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Find the first repeated word in a string

Given a string, Find the 1st repeated word in a string.

Examples: 

Input: “Ravi had been saying that he had been there”
Output: had

Input: “Ravi had been saying that”
Output: No Repetition

Input: “he had had he”
he

question source: https://www.geeksforgeeks.org/goldman-sachs-interview-experience-set-29-internship/amp/

Simple Approach : Start iterating from back and for every new word , store it in unordered map . For every word which has occurred more than one  , update ans to be that word , at last reverse ans and print it.

Implementation:




// Cpp program to find first repeated word in a string
#include<bits/stdc++.h>
using namespace std;
void solve(string s)
{
    unordered_map<string,int> mp;  // to store occurrences of word
    string t="",ans="";
    // traversing from back makes sure that we get the word which repeats first as ans
    for(int i=s.length()-1;i>=0;i--)
    {
        // if char present , then add that in temp word string t
        if(s[i]!=' ')
        {
            t+=s[i];
             
        }
        // if space is there then this word t needs to stored in map
        else
        {
            mp[t]++;
            // if that string t has occurred previously then it is a possible ans
            if(mp[t]>1)
               ans=t;
            // set t as empty for again new word  
            t="";
             
        }
    }
     
    // first word like "he" needs to be mapped
            mp[t]++;
            if(mp[t]>1)
               ans=t;
                           
    if(ans!="")
    {
        // reverse ans string as it has characters in reverse order
        reverse(ans.begin(),ans.end());
        cout<<ans<<'\n';
    }
    else
    cout<<"No Repetition\n";
}
int main()
{
    string u="Ravi had been saying that he had been there";
    string v="Ravi had been saying that";
    string w="he had had he";
    solve(u);
    solve(v);
    solve(w);
     
     
    return 0;
     
}




import java.util.*;
 
public class GFG {
  // Java  program to find first repeated word in a string
  public static void solve(String s)
  {
    HashMap<String, Integer> mp
      = new HashMap<String,
    Integer>(); // to store
    // occurrences of word
    String t = "";
    String ans = "";
    // traversing from back makes sure that we get the
    // word which repeats first as ans
    for (int i = s.length() - 1; i >= 0; i--) {
      // if char present , then add that in temp word
      // string t
      if (s.charAt(i) != ' ') {
        t += s.charAt(i);
      }
      // if space is there then this word t needs to
      // stored in map
      else {
        if (!mp.containsKey(t)) {
          mp.put(t, 1);
        }
        else {
          mp.put(t, mp.get(t) + 1);
        }
 
        // if that string t has occurred previously
        // then it is a possible ans
        if (mp.get(t) > 1) {
          ans = t;
        }
        // set t as empty for again new word
        t = "";
      }
    }
 
    // first word like "he" needs to be mapped
    if (!mp.containsKey(t)) {
      mp.put(t, 1);
    }
    else {
      mp.put(t, mp.get(t) + 1);
    }
    if (mp.get(t) > 1) {
      ans = t;
    }
 
    if (!ans.equals("")) {
      // reverse ans string as it has characters in
      // reverse order
      StringBuilder input1 = new StringBuilder();
 
      // append a string into StringBuilder input1
      input1.append(ans);
 
      // reverse StringBuilder input1
      input1.reverse();
 
      System.out.println(input1);
    }
    else {
      System.out.print("No Repetition\n");
    }
  }
  public static void main(String[] args)
  {
    String u
      = "Ravi had been saying that he had been there";
    String v = "Ravi had been saying that";
    String w = "he had had he";
    solve(u);
    solve(v);
    solve(w);
  }
}
 
// This code is contributed by Aarti_Rathi




# Python program to find first repeated word in a string
def solve(s):
 
    mp = {} # to store occurrences of word
    t = ""
    ans = ""
     
    # traversing from back makes sure that we get the word which repeats first as ans
    for i in range(len(s) - 1,-1,-1):
       
        # if char present , then add that in temp word string t
        if(s[i] != ' '):
            t += s[i]
         
        # if space is there then this word t needs to stored in map
        else:
         
            # if that string t has occurred previously then it is a possible ans
            if(t in mp):
                ans = t
            else:
                mp[t] = 1
             
            # set t as empty for again new word
            t = ""
             
    # first word like "he" needs to be mapped
    if(t in mp):
        ans=t
                         
    if(ans!=""):
 
        # reverse ans string as it has characters in reverse order
        ans = ans[::-1]
        print(ans)
    else:
        print("No Repetition")
 
# driver code
u = "Ravi had been saying that he had been there"
v = "Ravi had been saying that"
w = "he had had he"
solve(u)
solve(v)
solve(w)
 
# This code is contributed by shinjanpatra




// C# program to find first repeated word in a string
using System;
using System.Collections.Generic;
 
class GFG {
  static void solve(string s)
  {
    Dictionary<string, int> mp = new Dictionary<
      string, int>(); // to store occurrences of word
    string t = "";
    string ans = "";
    // traversing from back makes sure that we get the
    // word which repeats first as ans
    for (int i = s.Length - 1; i >= 0; i--) {
      // if char present , then add that in temp word
      // string t
      if (s[i] != ' ') {
        t += s[i];
      }
      // if space is there then this word t needs to
      // stored in map
      else {
        if (mp.ContainsKey(t)) {
          mp[t] += 1;
        }
        else {
          mp.Add(t, 1);
        }
        // if that string t has occurred previously
        // then it is a possible ans
        if (mp[t] > 1) {
          ans = t;
        }
        // set t as empty for again new word
        t = "";
      }
    }
 
    // first word like "he" needs to be mapped
    if (mp.ContainsKey(t)) {
      mp[t] += 1;
    }
    else {
      mp.Add(t, 1);
    }
 
    if (mp[t] > 1) {
      ans = t;
    }
 
    if (ans != "") {
      // reverse ans string as it has characters in
      // reverse order
      char[] charArray = ans.ToCharArray();
      Array.Reverse(charArray);
      Console.WriteLine(new string(charArray));
    }
    else {
      Console.Write("No Repetition\n");
    }
  }
  public static void Main()
  {
    string u
      = "Ravi had been saying that he had been there";
    string v = "Ravi had been saying that";
    string w = "he had had he";
    solve(u);
    solve(v);
    solve(w);
  }
}
 
// This code is contributed by Aarti_Rathi




<script>
// JavaScript program to find first repeated word in a string
 
function solve(s)
{
    let mp = new Map();  // to store occurrences of word
    let t = "";
    let ans = "";
     
    // traversing from back makes sure that we get the word which repeats first as ans
    for(let i = s.length - 1; i >= 0; i--)
    {
        // if char present , then add that in temp word string t
        if(s[i] != ' ')
        {
            t += s[i];
             
        }
         
        // if space is there then this word t needs to stored in map
        else
        {
         
            // if that string t has occurred previously then it is a possible ans
            if(mp.has(t))
               ans = t;
            else mp.set(t, 1)
             
            // set t as empty for again new word  
            t = "";
             
        }
    }
     
    // first word like "he" needs to be mapped
    if(mp.has(t)) ans=t;
                           
    if(ans!="")
    {
        // reverse ans string as it has characters in reverse order
        ans = [...ans].reverse().join("");
        document.write(ans);
    }
    else
    document.write("No Repetition");
}
 
// driver code
const u = "Ravi had been saying that he had been there";
const v = "Ravi had been saying that";
const w = "he had had he";
solve(u);
solve(v);
solve(w);
 
// This code is contributed by shinjanpatra
</script>

Output
had
No Repetition
he











Time complexity: O(N),because of for loop
Space Complexity: O(N),because of unordered_map/hashmap

Another Approach: The idea is to tokenize the string and store each word and its count in hashmap. Then traverse the string again and for each word of string, check its count in created hashmap. 

Implementation:




// CPP program for finding first repeated
// word in a string
#include <bits/stdc++.h>
using namespace std;
 
// returns first repeated word
string findFirstRepeated(string s)
{
    // break string into tokens
    // and then each string into set
    // if a word appeared before appears
    // again, return the word and break
 
    istringstream iss(s);
    string token;
 
    // hashmap for storing word and its count
    // in sentence
    unordered_map<string, int> setOfWords;
 
    // store all the words of string
    // and the count of word in hashmap
 
    while (getline(iss, token, ' ')) {
        if (setOfWords.find(token) != setOfWords.end())            
            setOfWords[token] += 1;  // word exists
        else
            // insert new word to set
            setOfWords.insert(make_pair(token, 1));       
    }
 
    // traverse again from first word of string s
    // to check if count of word is greater than 1
 
    // either take a new stream or store the words
    // in vector of strings in previous loop
    istringstream iss2(s);
    while (getline(iss2, token, ' ')) {
        int count = setOfWords[token];
        if (count > 1) {
            return token;
        }
    }
 
    return "NoRepetition";
}
 
// driver program
int main()
{
    string s("Ravi had been saying that he had been there");
    string firstWord = findFirstRepeated(s);
    if (firstWord != "NoRepetition")
        cout << "First repeated word :: "
             << firstWord << endl;
    else
        cout << "No Repetitionn";
    return 0;
}




// Java program for finding first repeated
// word in a string
import java.util.*;
 
class GFG{
     
    // returns first repeated word
    static String findFirstRepeated(String s)
    {
        // break string into tokens
        // and then each string into set
        // if a word appeared before appears
        // again, return the word and break
     
        String token[] = s.split(" ");
     
        // hashmap for storing word and its count
        // in sentence
        HashMap<String, Integer> setOfWords = new HashMap<String, Integer>();
     
        // store all the words of string
        // and the count of word in hashmap
     
        for (int i=0; i<token.length; i++) {
            if (setOfWords.containsKey(token[i]))           
                setOfWords.put(token[i], setOfWords.get(token[i]) + 1); // word exists
            else
                // insert new word to set
                setOfWords.put(token[i], 1);   
        }
     
        // traverse again from first word of string s
        // to check if count of word is greater than 1
     
        // either take a new stream or store the words
        // in vector of strings in previous loop
        for (int i=0; i<token.length; i++) {
            int count = setOfWords.get(token[i]);
            if (count > 1) {
                return token[i];
            }
        }
     
        return "NoRepetition";
    }
     
    // driver program
    public static void main(String args[])
    {
        String s = "Ravi had been saying that he had been there";
        String firstWord = findFirstRepeated(s);
        if (!firstWord.equals("NoRepetition"))
            System.out.println("First repeated word :: " + firstWord);
        else
            System.out.println("No Repetitionn");
    }
}




class GFG:
    # returns first repeated word
    @staticmethod
    def findFirstRepeated(s):
        # break string into tokens
        # and then each string into set
        # if a word appeared before appears
        # again, return the word and break
        token = s.split(" ")
         
        # map for storing word and its count
        # in sentence
        setOfWords = {}
         
        # store all the words of string
        # and the count of word in map
        for i in range(len(token)):
            if token[i] in setOfWords:
                setOfWords[token[i]] += 1
            else:
                # insert new word to map
                setOfWords[token[i]] = 1
         
        # traverse again from first word of string s
        # to check if count of word is greater than 1
        # either take a new stream or store the words
        # in array of strings in previous loop
        for i in range(len(token)):
            count = setOfWords[token[i]]
            if count > 1:
                return token[i]
         
        return "NoRepetition"
     
    # driver program
    @staticmethod
    def main(args):
        s = "Ravi had been saying that he had been there"
        firstWord = GFG.findFirstRepeated(s)
        if firstWord != "NoRepetition":
            print("First repeated word :: " + firstWord)
        else:
            print("No Repetition")
     
GFG.main([])
 
 
# This code is contributed by adityashatmfh




// C# program for finding first repeated
// word in a string
using System;
using System.Collections.Generic;
using System.Collections;
using System.Linq;
 
class HelloWorld {
     
    // returns first repeated word
    public static string findFirstRepeated(string s)
    {
        // break string into tokens
        // and then each string into set
        // if a word appeared before appears
        // again, return the word and break
 
        string[] token = s.Split(" ");
 
        // hashmap for storing word and its count
        // in sentence
        Dictionary<string, int> setOfWords = new Dictionary<string,int>();
 
        // store all the words of string
        // and the count of word in map
        for (int i=0; i < token.Length; i++)
        {
            if (setOfWords.ContainsKey(token[i]) == true)
            {
                setOfWords[token[i]] = setOfWords[token[i]] + 1;
            }
            else
            {
                // insert new word to map
                setOfWords.Add(token[i], 1);
            }
        }
        // traverse again from first word of string s
        // to check if count of word is greater than 1
        // either take a new stream or store the words
        // in array of strings in previous loop
        for (int i=0; i < token.Length; i++)
        {
            int count = setOfWords[token[i]];
            if (count > 1)
            {
                return token[i];
            }
        }
        return "NoRepetition";
    }   
 
    static void Main() {
        string s = "Ravi had been saying that he had been there";
        string firstWord = findFirstRepeated(s);
        if (firstWord != "NoRepetition")
            Console.WriteLine("First repeated word :: "  + firstWord);
        else
            Console.WriteLine("No Repitition");
    }
}
 
// The code is contributed by Nidhi goel.




class GFG
{
    // returns first repeated word
    static findFirstRepeated(s)
    {
        // break string into tokens
        // and then each string into set
        // if a word appeared before appears
        // again, return the word and break
        var token = s.split(" ");
         
        // map for storing word and its count
        // in sentence
        var setOfWords = new Map();
        // store all the words of string
        // and the count of word in map
        for (let i=0; i < token.length; i++)
        {
            if (setOfWords.has(token[i]))
            {
                setOfWords.set(token[i],setOfWords.get(token[i]) + 1);
            }
            else
            {
                // insert new word to map
                setOfWords.set(token[i],1);
            }
        }
        // traverse again from first word of string s
        // to check if count of word is greater than 1
        // either take a new stream or store the words
        // in array of strings in previous loop
        for (let i=0; i < token.length; i++)
        {
            var count = setOfWords.get(token[i]);
            if (count > 1)
            {
                return token[i];
            }
        }
        return "NoRepetition";
    }
    // driver program
    static main(args)
    {
        var s = "Ravi had been saying that he had been there";
        var firstWord = GFG.findFirstRepeated(s);
        if (firstWord !== "NoRepetition")
        {
            console.log("First repeated word :: " + firstWord);
        }
        else
        {
            console.log("No Repetitionn");
        }
    }
}
GFG.main([]);
 
// This code is contributed by mukulsomukesh

Output
First repeated word :: had











Method #2: Using built in python functions:

Implementation::




#include <iostream>
#include <string>
#include <vector>
#include <unordered_map>
 
using namespace std;
 
string firstRepeatedWord(string sentence)
{
    // Splitting the string into words
    vector<string> words;
    size_t pos = 0;
    while ((pos = sentence.find(' ')) != string::npos)
    {
        string word = sentence.substr(0, pos);
        words.push_back(word);
        sentence.erase(0, pos + 1);
    }
    words.push_back(sentence); // Add the last word
 
    // Calculating the frequency of each word
    unordered_map<string, int> frequency;
    for (size_t i = 0; i < words.size(); i++)
    {
        string word = words[i];
        if (frequency.find(word) == frequency.end())
        {
            frequency[word] = 1;
        }
        else
        {
            frequency[word]++;
        }
    }
 
    // Traversing the list of words
    for (size_t i = 0; i < words.size(); i++)
    {
        string word = words[i];
        // Checking if the frequency is greater than 1
        if (frequency[word] > 1)
        {
            // Return the word
            return word;
        }
    }
 
    // If no repeated word is found
    return "No repeated word found";
}
 
int main()
{
    string sentence = "Vikram had been saying that he had been there";
    cout << firstRepeatedWord(sentence) << endl;
    return 0;
}
 
// This code is contributed by akshitaguprzj3




import java.util.*; 
 
class GFG
{
  public static String firstRepeatedWord(String sentence)
  {
    // splitting the string
    String[] lis = sentence.split(" ");
 
    // Calculating frequency of every word
    Map<String, Integer> frequency = new HashMap<>();
    for (int i = 0; i < lis.length; i++)
    {
      String word = lis[i];
      if (!frequency.containsKey(word))
      {
        frequency.put(word, 1);
      }
      else
      {
        frequency.put(word, frequency.get(word) + 1);
      }
    }
 
    // Traversing the list of words
    for (int i = 0; i < lis.length; i++)
    {
      String word = lis[i];
      // checking if frequency is greater than 1
      if (frequency.get(word) > 1)
      {
        // return the word
        return word;
      }
    }
 
    // if no repeated word is found
    return "No repeated word found";
  }
 
  // Driver code
  public static void main(String[] args)
  {
    String sentence = "Vikram had been saying that he had been there";
    System.out.println(firstRepeatedWord(sentence));
  }
 
}








using System;
using System.Collections.Generic;
 
class Program
{
    static string FirstRepeatedWord(string sentence)
    {
        // Splitting the string into words
        List<string> words = new List<string>();
        int pos;
        while ((pos = sentence.IndexOf(' ')) != -1)
        {
            string word = sentence.Substring(0, pos);
            words.Add(word);
            sentence = sentence.Remove(0, pos + 1);
        }
        words.Add(sentence); // Add the last word
 
        // Calculating the frequency of each word
        Dictionary<string, int> frequency = new Dictionary<string, int>();
        foreach (string word in words)
        {
            if (!frequency.ContainsKey(word))
            {
                frequency[word] = 1;
            }
            else
            {
                frequency[word]++;
            }
        }
 
        // Traversing the list of words
        foreach (string word in words)
        {
            // Checking if the frequency is greater than 1
            if (frequency[word] > 1)
            {
                // Return the word
                return word;
            }
        }
 
        // If no repeated word is found
        return "No repeated word found";
    }
 
    static void Main()
    {
        string sentence = "Vikram had been saying that he had been there";
        Console.WriteLine(FirstRepeatedWord(sentence));
    }
}
 
// This code is contributed by shivamgupta0987654321





Output
had











Another Approach: 

Instead of tracking the counts for a specific token(word), we can keep track of the first occurrence of the token(word) using an unordered map. This would not require any extra loop to traverse in a hashmap or a string to find the repeated string. Thus, it eventually transforms the time complexity from O(2*n) to O(n) while the space complexity remains the same.

Implementation:




// CPP program to find first repeated word in a string.
#include <bits/stdc++.h>
using namespace std;
 
void solve(string s)
{
    int n = s.size(); // size of the string.
    unordered_map<string, int>
        mp; // To store first occurrence of a word.
    string ans = "", t = "";
    int min_idx = INT_MAX; // To get minimum occurrence in
                           // the given string.
 
    int i = 0,
        j = 0; // iterators. i -> initial index of a word
               //            j -> final index of a word.
 
    // loop to traverse in a string and to find out each
    // repeated word whose occurrence is minimum.
    while (j <= n) {
 
        // If found an entire word then check if it is
        // repeated or not using unordered_map.
        if (s[j] == ' ' || j == n) {
            if (mp[t] == 0) { // Store the first occurrence
                              // of a word.
                mp[t] = i + 1;
            }
            else { // If there is a Repetition then check
                   // for minimum occurrence of the word.
                if (min_idx > mp[t]) {
                    min_idx = mp[t];
                    ans = t;
                }
            }
 
            // Shift the pointers.
            t = "";
            i = j + 1;
            j = i;
        }
        else {
            t += s[j];
            j++;
        }
    }
 
    // If ans is of empty string then this signifies that
    // there is no Repetition.
    if (ans == "")
        cout << "No Repetition" << endl;
    else
        cout << ans << endl;
}
 
int main()
{
    string s1
        = "Ravi had been saying that he had been there";
    string s2 = "Ravi had been saying that";
    string s3 = "he had had he";
 
    solve(s1);
    solve(s2);
    solve(s3);
 
    return 0;
}




// Java program to find first repeated word in a string.
 
import java.io.*;
import java.util.*;
 
import java.util.*;
 
public class Main {
     
    public static void solve(String s) {
        int n = s.length(); // size of the string.
        Map<String, Integer> mp = new HashMap<>(); // To store first occurrence of a word.
        String ans = "", t = "";
        int min_idx = Integer.MAX_VALUE; // To get minimum occurrence in the given string.
 
        int i = 0, j = 0; // iterators. i -> initial index of a word, j -> final index of a word.
 
        // loop to traverse in a string and to find out each
        // repeated word whose occurrence is minimum.
        while (j <= n) {
 
            // If found an entire word then check if it is
            // repeated or not using unordered_map.
            if (j == n || s.charAt(j) == ' ') {
                if (mp.getOrDefault(t, 0) == 0) { // Store the first occurrence of a word.
                    mp.put(t, i + 1);
                }
                else { // If there is a repetition then check for minimum occurrence of the word.
                    if (min_idx > mp.get(t)) {
                        min_idx = mp.get(t);
                        ans = t;
                    }
                }
 
                // Shift the pointers.
                t = "";
                i = j + 1;
                j = i;
            }
            else {
                t += s.charAt(j);
                j++;
            }
        }
 
        // If ans is an empty string then this signifies that
        // there is no Repetition.
        if (ans.equals(""))
            System.out.println("No Repetition");
        else
            System.out.println(ans);
    }
 
    public static void main(String[] args) {
        String s1 = "Ravi had been saying that he had been there";
        String s2 = "Ravi had been saying that";
        String s3 = "he had had he";
 
        solve(s1);
        solve(s2);
        solve(s3);
    }
}
 
// The code is contributed by Arushi Goel.




def find_first_repeated_word(s):
    n = len(s)  # Size of the string.
    mp = {}  # To store the first occurrence of a word.
    ans = ""
    t = ""
    min_idx = float('inf'# To get the minimum occurrence in the given string.
 
    i = 0
    j = 0  # Iterators: i -> initial index of a word, j -> final index of a word.
 
    # Loop to traverse the string and find each repeated word whose occurrence is minimum.
    while j <= n:
        # If found an entire word, then check if it is repeated or not using a dictionary.
        if j == n or s[j] == ' ':
            if t not in mp:
                mp[t] = i + 1  # Store the first occurrence of a word.
            else:
                # If there is a repetition, then check for the minimum occurrence of the word.
                if min_idx > mp[t]:
                    min_idx = mp[t]
                    ans = t
 
            # Shift the pointers.
            t = ""
            i = j + 1
            j = i
        else:
            t += s[j]
            j += 1
 
    # If ans is an empty string, it signifies that there is no repetition.
    if ans == "":
        print("No Repetition")
    else:
        print(ans)
 
# Test cases
s1 = "Ravi had been saying that he had been there"
s2 = "Ravi had been saying that"
s3 = "he had had he"
 
find_first_repeated_word(s1)
find_first_repeated_word(s2)
find_first_repeated_word(s3)
 
# This code is contributed by shivamgupta310570




using System;
using System.Collections.Generic;
 
public class MainClass
{
    public static void Solve(string s)
    {
        int n = s.Length; // size of the string.
        Dictionary<string, int> mp = new Dictionary<string, int>(); // To store first occurrence of a word.
        string ans = "", t = "";
        int min_idx = int.MaxValue; // To get minimum occurrence in the given string.
 
        int i = 0, j = 0; // iterators. i -> initial index of a word, j -> final index of a word.
 
        // loop to traverse in a string and to find out each
        // repeated word whose occurrence is minimum.
        while (j <= n)
        {
            // If found an entire word then check if it is
            // repeated or not using Dictionary.
            if (j == n || s[j] == ' ')
            {
                if (!mp.ContainsKey(t))
                {
                    mp[t] = i + 1; // Store the first occurrence of a word.
                }
                else
                {
                    // If there is a repetition then check for minimum occurrence of the word.
                    if (min_idx > mp[t])
                    {
                        min_idx = mp[t];
                        ans = t;
                    }
                }
 
                // Shift the pointers.
                t = "";
                i = j + 1;
                j = i;
            }
            else
            {
                t += s[j];
                j++;
            }
        }
 
        // If ans is an empty string then this signifies that
        // there is no Repetition.
        if (ans == "")
        {
            Console.WriteLine("No Repetition");
        }
        else
        {
            Console.WriteLine(ans);
        }
    }
 
    public static void Main(string[] args)
    {
        string s1 = "Ravi had been saying that he had been there";
        string s2 = "Ravi had been saying that";
        string s3 = "he had had he";
 
        Solve(s1);
        Solve(s2);
        Solve(s3);
    }
}





Output
had
No Repetition
he











This article is contributed by Aarti_Rathi and Mandeep Singh.


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