Repeated Character Whose First Appearance is Leftmost

Given a string, find the repeated character present first in the string.

Examples:

Input  : geeksforgeeks
Output : g
(mind that it will be g, not e.)

Input  : abcdabcd
Output : a

Input  : abcd
Output : -1
No character repeats

Asked in: Goldman Sachs internship

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

We have discussed different approaches in Find repeated character present first in a string.

How to solve this problem using one traversal of input string?

Method 1 (Traversing from Left to Right) We traverse the string from left to right. We keep track of the leftmost index of every character. If a character repeats, we compare its leftmsot index with current result and update the result if result is greater

C++

// CPP program to find first repeating 
// character 
#include <bits/stdc++.h> 
using namespace std; 
#define NO_OF_CHARS 256 
  
/* The function returns index of the first  
repeating character in a string. If  
all characters are repeating then  
returns -1 */
int firstRepeating(string& str) 
{ 
    // Initialize leftmost index of every 
    // character as -1. 
    int firstIndex[NO_OF_CHARS]; 
    for (int i = 0; i < NO_OF_CHARS; i++) 
        firstIndex[i] = -1; 
  
    // Traverse from left and update result 
    // if we see a repeating character whose 
    // first index is smaller than current 
    // result. 
    int res = INT_MAX; 
    for (int i = 0; i < str.length(); i++) { 
        if (firstIndex[str[i]] == -1) 
           firstIndex[str[i]] = i; 
        else
           res = min(res, firstIndex[str[i]]); 
    } 
  
    return (res == INT_MAX) ? -1 : res; 
} 
  
/* Driver program to test above function */
int main() 
{ 
    string str = "geeksforgeeks"; 
    int index = firstRepeating(str); 
    if (index == -1) 
        printf("Either all characters are "
               "distinct or string is empty"); 
    else
        printf("First Repeating character"
               " is %c", 
               str[index]); 
    return 0; 
} 

Java

// Java program to find first repeating 
// character 
  
class GFG  
{ 
  
    static int NO_OF_CHARS = 256; 
  
    /* The function returns index of the first  
    repeating character in a string. If  
    all characters are repeating then  
    returns -1 */
    static int firstRepeating(char[] str)  
    { 
        // Initialize leftmost index of every 
        // character as -1. 
        int[] firstIndex = new int[NO_OF_CHARS]; 
        for (int i = 0; i < NO_OF_CHARS; i++) 
        { 
            firstIndex[i] = -1; 
        } 
  
        // Traverse from left and update result 
        // if we see a repeating character whose 
        // first index is smaller than current 
        // result. 
        int res = Integer.MAX_VALUE; 
        for (int i = 0; i < str.length; i++)  
        { 
            if (firstIndex[str[i]] == -1)  
            { 
                firstIndex[str[i]] = i; 
            }  
            else
            { 
                res = Math.min(res, firstIndex[str[i]]); 
            } 
        } 
  
        return (res == Integer.MAX_VALUE) ? -1 : res; 
    } 
  
    /* Driver code */
    public static void main(String[] args) 
    { 
        char[] str = "geeksforgeeks".toCharArray(); 
        int index = firstRepeating(str); 
        if (index == -1) 
        { 
            System.out.printf("Either all characters are "
                    + "distinct or string is empty"); 
        }  
        else
        { 
            System.out.printf("First Repeating character"
                    + " is %c", str[index]); 
        } 
    } 
} 
  
// This code has been contributed by 29AjayKumar 

Python3

# Python 3 program to find first repeating 
# character 
import sys 
  
NO_OF_CHARS = 256
  
# The function returns index of the first  
# repeating character in a string. If  
# all characters are repeating then  
# returns -1 */ 
def firstRepeating(str): 
      
    # Initialize leftmost index of every 
    # character as -1. 
    firstIndex = [0 for i in range(NO_OF_CHARS)] 
    for i in range(NO_OF_CHARS): 
        firstIndex[i] = -1
  
    # Traverse from left and update result 
    # if we see a repeating character whose 
    # first index is smaller than current 
    # result. 
    res = sys.maxsize 
    for i in range(len(str)): 
        if (firstIndex[ord(str[i])] == -1): 
            firstIndex[ord(str[i])] = i 
        else: 
            res = min(res, firstIndex[ord(str[i])]) 
  
    if res == sys.maxsize: 
        return -1
    else: 
        return res 
  
# Driver function 
if __name__ == '__main__': 
    str = "geeksforgeeks"
    index = firstRepeating(str) 
    if (index == -1): 
        print("Either all characters are distinct or string is empty") 
    else: 
        print("First Repeating character is",str[index]) 
          
# This code is contributed by 
# Surendra_Gangwar 

C#

// C# program to find first repeating 
// character 
using System; 
using System.Collections.Generic; 
      
class GFG  
{ 
  
    static int NO_OF_CHARS = 256; 
  
    /* The function returns index of the first  
    repeating character in a string. If  
    all characters are repeating then  
    returns -1 */
    static int firstRepeating(char[] str)  
    { 
        // Initialize leftmost index of every 
        // character as -1. 
        int[] firstIndex = new int[NO_OF_CHARS]; 
        for (int i = 0; i < NO_OF_CHARS; i++) 
        { 
            firstIndex[i] = -1; 
        } 
  
        // Traverse from left and update result 
        // if we see a repeating character whose 
        // first index is smaller than current 
        // result. 
        int res = int.MaxValue; 
        for (int i = 0; i < str.Length; i++)  
        { 
            if (firstIndex[str[i]] == -1)  
            { 
                firstIndex[str[i]] = i; 
            }  
            else
            { 
                res = Math.Min(res, firstIndex[str[i]]); 
            } 
        } 
  
        return (res == int.MaxValue) ? -1 : res; 
    } 
  
    /* Driver code */
    public static void Main(String[] args) 
    { 
        char[] str = "geeksforgeeks".ToCharArray(); 
        int index = firstRepeating(str); 
        if (index == -1) 
        { 
            Console.Write("Either all characters are "
                    + "distinct or string is empty"); 
        }  
        else
        { 
            Console.Write("First Repeating character"
                    + " is {0}", str[index]); 
        } 
    } 
} 
  
// This code is contributed by Princi Singh 

Output:

First Repeating character is g

Time Complexity : O(n). It does only one traversal of input string.
Auxiliary Space : O(1)

Method 2 (Traversing Right to Left) We traverse the string from right to left. We keep track of the visited characters. If a character repeats, we update the result.

C++

// CPP program to find first repeating 
// character 
#include <bits/stdc++.h> 
using namespace std; 
#define NO_OF_CHARS 256 
  
/* The function returns index of the first  
repeating character in a string. If  
all characters are repeating then  
returns -1 */
int firstRepeating(string& str) 
{ 
    // Mark all characters as not visited 
    bool visited[NO_OF_CHARS]; 
    for (int i = 0; i < NO_OF_CHARS; i++) 
        visited[i] = false; 
  
    // Traverse from right and update res as soon 
    // as we see a visited character. 
    int res = -1; 
    for (int i = str.length() - 1; i >= 0; i--) { 
        if (visited[str[i]] == false) 
            visited[str[i]] = true; 
        else
            res = i; 
    } 
  
    return res; 
} 
  
/* Driver program to test above function */
int main() 
{ 
    string str = "geeksforgeeks"; 
    int index = firstRepeating(str); 
    if (index == -1) 
        printf("Either all characters are "
               "distinct or string is empty"); 
    else
        printf("First Repeating character"
               " is %c", 
               str[index]); 
    return 0; 
} 

Java

// Java program to find first repeating 
// character 
import java.util.*; 
  
class GFG  
{ 
  
static int NO_OF_CHARS= 256; 
  
/* The function returns index of the first  
repeating character in a string. If  
all characters are repeating then  
returns -1 */
static int firstRepeating(String str) 
{ 
    // Mark all characters as not visited 
    boolean []visited = new boolean[NO_OF_CHARS]; 
    for (int i = 0; i < NO_OF_CHARS; i++) 
        visited[i] = false; 
  
    // Traverse from right and update res as soon 
    // as we see a visited character. 
    int res = -1; 
    for (int i = str.length() - 1; i >= 0; i--)  
    { 
        if (visited[str.charAt(i)] == false) 
            visited[str.charAt(i)] = true; 
        else
            res = i; 
    } 
  
    return res; 
} 
  
/* Driver code */
public static void main(String[] args)  
{ 
    String str = "geeksforgeeks"; 
    int index = firstRepeating(str); 
    if (index == -1) 
        System.out.printf("Either all characters are "
            +"distinct or string is empty"); 
    else
        System.out.printf("First Repeating character"
            +" is %c", 
            str.charAt(index)); 
} 
} 
  
// This code contributed by Rajput-Ji 

Python3

# Python3 program to find first  
# repeating character  
  
NO_OF_CHARS = 256
  
""" The function returns index of the first  
repeating character in a string. If  
all characters are repeating then  
returns -1 """
  
def firstRepeating(string) : 
      
    # Mark all characters as not visited  
    visited = [False] * NO_OF_CHARS; 
      
    for i in range(NO_OF_CHARS) : 
        visited[i] = False; 
          
    # Traverse from right and update res as soon  
    # as we see a visited character.  
    res = -1; 
    for i in range(len(string)-1, -1 , -1) : 
        if (visited[string.index(string[i])] == False): 
            visited[string.index(string[i])] = True; 
          
        else: 
            res = i; 
      
    return res;  
      
      
# Driver program to test above function  
if __name__ == "__main__" :  
  
    string = "geeksforgeeks";  
    index = firstRepeating(string);  
      
    if (index == -1) : 
        print("Either all characters are" , 
        "distinct or string is empty");  
    else : 
        print("First Repeating character is:", string[index]);  
          
# This code is contributed by AnkitRai01 

C#

// C# program to find first repeating 
// character 
using System;  
      
class GFG  
{ 
  
static int NO_OF_CHARS = 256; 
  
/* The function returns index of the first  
repeating character in a string. If  
all characters are repeating then  
returns -1 */
static int firstRepeating(String str) 
{ 
    // Mark all characters as not visited 
    Boolean []visited = new Boolean[NO_OF_CHARS]; 
    for (int i = 0; i < NO_OF_CHARS; i++) 
        visited[i] = false; 
  
    // Traverse from right and update res as soon 
    // as we see a visited character. 
    int res = -1; 
    for (int i = str.Length - 1; i >= 0; i--)  
    { 
        if (visited[str[i]] == false) 
            visited[str[i]] = true; 
        else
            res = i; 
    } 
  
    return res; 
} 
  
// Driver Code 
public static void Main(String[] args)  
{ 
    String str = "geeksforgeeks"; 
    int index = firstRepeating(str); 
    if (index == -1) 
        Console.Write("Either all characters are " +  
                      "distinct or string is empty"); 
    else
        Console.Write("First Repeating character" +  
                      " is {0}", str[index]); 
} 
} 
  
// This code is contributed by PrinciRaj1992  

Output:

First Repeating character is g

Time Complexity : O(n). It does only one traversal of input string.
Auxiliary Space : O(1)

The method 2 is better than method 1 as it does fewer comparisons.

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My Personal Notes

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

Python3

C#

C++

Java

Python3

C#

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