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# First non-repeating character using one traversal of string | Set 2

Given a string, find the first non-repeating character in it. For example, if the input string is “GeeksforGeeks”, then output should be ‘f’ and if input string is “GeeksQuiz”, then output should be ‘G’.

We have discussed two solutions in Given a string, find its first non-repeating character . In this post, a further optimized solution (over method 2 of previous post) is discussed. The idea is to optimize space. Instead of using a pair to store count and index, we use single element that stores index if an element appears once, else stores a negative value.

Implementation:

## C++

 // CPP program to find first non-repeating// character using 1D array and one traversal.#include using namespace std;#define NO_OF_CHARS 256 /* The function returns index of the firstnon-repeating character in a string. Ifall characters are repeating thenreturns INT_MAX */int firstNonRepeating(char* str){       // Initialize all characters as    // absent.    int arr[NO_OF_CHARS];    for (int i = 0; i < NO_OF_CHARS; i++)        arr[i] = -1;     // After below loop, the value of    // arr[x] is going to be index of    // of x if x appears only once. Else    // the value is going to be either    // -1 or -2.    for (int i = 0; str[i]; i++) {        if (arr[str[i]] == -1)            arr[str[i]] = i;        else            arr[str[i]] = -2;    }     int res = INT_MAX;    for (int i = 0; i < NO_OF_CHARS; i++)         // If this character occurs only        // once and appears before the        // current result, then update the        // result        if (arr[i] >= 0)            res = min(res, arr[i]);     return res;} /* Driver program to test above function */int main(){    char str[] = "geeksforgeeks";    int index = firstNonRepeating(str);    if (index == INT_MAX)        cout <<"Either all characters are "               "repeating or string is empty";    else        cout <<"First non-repeating character"               " is "<< str[index];    return 0;} // This code is contributed by shivanisinghss210

## C

 // CPP program to find first non-repeating// character using 1D array and one traversal.#include #include #include #define NO_OF_CHARS 256 /* The function returns index of the firstnon-repeating character in a string. Ifall characters are repeating thenreturns INT_MAX */int firstNonRepeating(char* str){    // Initialize all characters as    // absent.    int arr[NO_OF_CHARS];    for (int i = 0; i < NO_OF_CHARS; i++)        arr[i] = -1;     // After below loop, the value of    // arr[x] is going to be index of    // of x if x appears only once. Else    // the value is going to be either    // -1 or -2.    for (int i = 0; str[i]; i++) {        if (arr[str[i]] == -1)            arr[str[i]] = i;        else            arr[str[i]] = -2;    }     int res = INT_MAX;    for (int i = 0; i < NO_OF_CHARS; i++)         // If this character occurs only        // once and appears before the        // current result, then update the        // result        if (arr[i] >= 0)            res = min(res, arr[i]);     return res;} /* Driver program to test above function */int main(){    char str[] = "geeksforgeeks";    int index = firstNonRepeating(str);    if (index == INT_MAX)        printf("Either all characters are "               "repeating or string is empty");    else        printf("First non-repeating character"               " is %c", str[index]);    return 0;}

## Java

 // Java program to find first// non-repeating character// using 1D array and one// traversal.import java.io.*;import java.util.*;import java.lang.*; class GFG{/* The function returns indexof the first non-repeatingcharacter in a string. Ifall characters are repeatingthen returns INT_MAX */static int firstNonRepeating(String str){    int NO_OF_CHARS = 256;         // Initialize all characters    // as absent.    int arr[] = new int[NO_OF_CHARS];    for (int i = 0;             i < NO_OF_CHARS; i++)        arr[i] = -1;     // After below loop, the    // value of arr[x] is going    // to be index of x if x    // appears only once. Else    // the value is going to be    // either -1 or -2.    for (int i = 0;             i < str.length(); i++)    {        if (arr[str.charAt(i)] == -1)            arr[str.charAt(i)] = i;        else            arr[str.charAt(i)] = -2;    }     int res = Integer.MAX_VALUE;    for (int i = 0; i < NO_OF_CHARS; i++)         // If this character occurs        // only once and appears before        // the current result, then        // update the result        if (arr[i] >= 0)            res = Math.min(res, arr[i]);     return res;} // Driver Codepublic static void main(String args[]){    String str = "geeksforgeeks";         int index = firstNonRepeating(str);    if (index == Integer.MAX_VALUE)        System.out.print("Either all characters are " +                       "repeating or string is empty");    else        System.out.print("First non-repeating character"+                             " is " + str.charAt(index));}}

## Python3

 '''Python3 implementation to find non repeating character using1D array and one traversal'''import math as mt NO_OF_CHARS = 256 '''The function returns index of the firstnon-repeating character in a string. Ifall characters are repeating thenreturns INT_MAX ''' def firstNonRepeating(string):    #initialize all character as absent         arr=[-1 for i in range(NO_OF_CHARS)]         '''    After below loop, the value of    arr[x] is going to be index of    of x if x appears only once. Else    the value is going to be either    -1 or -2.'''         for i in range(len(string)):        if arr[ord(string[i])]==-1:            arr[ord(string[i])]=i        else:            arr[ord(string[i])]=-2    res=10**18         for i in range(NO_OF_CHARS):        '''        If this character occurs only        once and appears before the        current result, then update the        result'''        if arr[i]>=0:            res=min(res,arr[i])    return res #Driver program to test above function string="geeksforgeeks" index=firstNonRepeating(string) if index==10**18:    print("Either all characters are repeating or string is empty")else:    print("First non-repeating character is",string[index]) #this code is contributed by Mohit Kumar 29

## C#

 // C# program to find first// non-repeating character// using 1D array and one// traversal.using System; class GFG{    /* The function returns index    of the first non-repeating    character in a string. If    all characters are repeating    then returns INT_MAX */    static int firstNonRepeating(String str)    {        int NO_OF_CHARS = 256;         // Initialize all characters        // as absent.        int []arr = new int[NO_OF_CHARS];        for (int i = 0; i < NO_OF_CHARS; i++)            arr[i] = -1;         // After below loop, the        // value of arr[x] is going        // to be index of x if x        // appears only once. Else        // the value is going to be        // either -1 or -2.        for (int i = 0; i < str.Length; i++)        {            if (arr[str[i]] == -1)                arr[str[i]] = i;            else                arr[str[i]] = -2;        }         int res = int.MaxValue;        for (int i = 0; i < NO_OF_CHARS; i++)             // If this character occurs            // only once and appears before            // the current result, then            // update the result            if (arr[i] >= 0)                res = Math.Min(res, arr[i]);         return res;    }     // Driver Code    public static void Main()    {        String str = "geeksforgeeks";        int index = firstNonRepeating(str);        if (index == int.MaxValue)            Console.Write("Either all characters are " +                        "repeating or string is empty");        else            Console.Write("First non-repeating character"+                                " is " + str[index]);    }} // This code is contributed by Rajput-Ji

## Javascript



Output

First non-repeating character is f

Time Complexity: O(n)
Auxiliary Space: O(1)

Alternate Implementation:

This is coded using a HashMap or Hashing Technique.

If the element(or key) repeats in the string the HashMap (or Dictionary) will change the value of that key to None.

This way we will later on only be finding keys whose value is “not None”.

Implementation:

## C++

 // C++ implementation of// above approach#include #include using namespace std; // Function to make a frequency// dictionary of characters in a string.unordered_map makeHashMap(string string){    // Create an empty HashMap    unordered_map d1;      // To store each character and its frequency    char c;      // Traversing through the string    for (int i = 0; i < string.length(); i++) {          // Extracting each character        c = string[i];          // If the character has occurred before        if (d1.find(c) != d1.end()) {              // Increment its frequency            d1++;        }          // If the character has not occurred before        else            d1 = 1;    }      // Update value of each key such that    // if frequency of  frequency of  a key    // is equal to 1, then it is set to 0,    // else set the value equal to None    for (auto it = d1.begin(); it != d1.end(); it++)    {        if (it->second == 1)            it->second = 0;        else            it->second = -1;    }      return d1;}  // Function to return the first// non repeating characterchar firstNotRepeatingCharacter(string s){    // Make a frequency dictionary    unordered_map d = makeHashMap(s);      // Variable to store the result    char nonRep = '.';      // Traversing through the string only once    for (int i = 0; i < s.length(); i++) {        char c = s[i];          // If the character's value in the        // frequency dictionary is "not None"        if (d == 0) {              // Store it in the result            nonRep = c;              // Break out of the loop            break;        }    }      // If no non-repeating character is found    if (nonRep == '.')        return '_';    else        return nonRep;}  // Driver Codeint main(){    string s = "bbcdcca";    cout << firstNotRepeatingCharacter(s);    return 0;}

## Java

 // Java implementation of// above approachimport java.util.HashMap; class GFG {      // Function to make a frequency    // dictionary of characters in a string.    public static HashMap makeHashMap(String string)    {        // Create an empty HashMap        HashMap d1 =                    new HashMap();          // To store each character and its frequency        char c;          // Traversing through the string        for (int i = 0; i < string.length(); i++) {              // Extracting each character            c = string.charAt(i);              // If the character has occurred before            if (d1.containsKey(c)) {                  // Increment its frequency                Integer oldValue = d1.get(c);                d1.put(c, oldValue + 1);            }              // If the character has not occurred before            else                d1.put(c, 1);        }          // Update value of each key such that        // if frequency of  frequency of  a key        // is equal to 1, then it is set to 0,        // else set the value equal to None        for (Character key : d1.keySet()) {            if (d1.get(key) == 1)                d1.put(key, 0);            else                d1.put(key, null);        }          return d1;    }      // Function to return the first    // non repeating character    public static Character firstNotRepeatingCharacter(String s)    {        // Make a frequency dictionary        HashMap d = makeHashMap(s);          // Variable to store the result        Character nonRep = null;          // Traversing through the string only once        for (int i = 0; i < s.length(); i++) {            Character c = s.charAt(i);              // If the character's value in the            // frequency dictionary is "not None"            if (d.get(c) != null) {                  // Store it in the result                nonRep = c;                  // Break out of the loop                break;            }        }          // If no non-repeating character is found        if (nonRep == null)            return '_';        else            return nonRep;    }      // Driver Code    public static void main(String args[])    {        String s = "bbcdcca";        System.out.println(firstNotRepeatingCharacter(s));    }}

## Python3

 # Python implementation of# above approachfrom collections import Counter  def makeHashMap(string):     # Use Counter to make a frequency    # dictionary of characters in a string.     d1 = Counter(string)     # Update value of each key such that    # if frequency of  frequency of  a key    # is equal to 1, then it is set to 0,    # else set the value equal to None    d1 = {(key): (0 if d1[key] == 1 else None)          for key, value in d1.items()}     return d1  def firstNotRepeatingCharacter(s):     d = makeHashMap(s)     # variable to store the first    # non repeating character.    nonRep = None     # Traversing through the string only once.    for i in s:        if d[i] is not None:             '''            As soon as the first character in the string is            found whose value in the HashMap is "not None",            that is our first non-repeating character.            So we store it in nonRep and break out of the            loop. Thus saving on time.            '''            nonRep = i            break     if nonRep is not None:        return nonRep    else:         # If there are no non-repeating characters        # then "_" is returned        return str("_")  # Driver Codeprint(firstNotRepeatingCharacter('bbcdcca')) # This code is contributed by Vivek Nigam (@viveknigam300)

## C#

 using System;using System.Collections.Generic; public class Program {    // Function to create a Dictionary with characters and    // their frequency    public static Dictionary    MakeHashMap(string s)    {        // Initializing an empty dictionary        var d1 = new Dictionary();        // Iterating through each character in the string        foreach(char c in s)        {            // If the character is already in the            // dictionary, increment its frequency            if (d1.ContainsKey(c)) {                d1++;            }            // Else add the character with frequency 1 to            // the dictionary            else {                d1 = 1;            }        }         // Initializing another empty dictionary for marking        // repeated characters        var d2 = new Dictionary();         // Iterating through each key-value pair in the        // first dictionary        foreach(var kvp in d1)        {            // If the character occurs only once, set its            // value to 0 in the second dictionary            if (kvp.Value == 1) {                d2[kvp.Key] = 0;            }            // Else set its value to -1 in the second            // dictionary            else {                d2[kvp.Key] = -1;            }        }         // Return the second dictionary        return d2;    }     // Function to find the first non-repeating character in    // the given string    public static char FirstNotRepeatingCharacter(string s)    {        // Create a dictionary with the frequency of each        // character        var d = MakeHashMap(s);         // Variable to store the first non-repeating        // character        char nonRep = '.';         // Iterate through each character in the string        foreach(char c in s)        {            // If the frequency of the character is 0, it is            // non-repeating            if (d == 0) {                // Store the non-repeating character and                // break out of the loop                nonRep = c;                break;            }        }         // If no non-repeating character is found, return        // '_'        if (nonRep == '.') {            return '_';        }        else {            // Else return the first non-repeating character            return nonRep;        }    }     // Main method    public static void Main()    {        string s = "bbcdcca";        Console.WriteLine(FirstNotRepeatingCharacter(s));    }}

## Javascript

 // Javascript implementation of// above approachfunction makeHashMap(string) {     // Use Map to make a frequency    // dictionary of characters in a string.    let d1 = new Map();    for (let i = 0; i < string.length; i++) {        let char = string.charAt(i);        if (d1.has(char)) {            d1.set(char, d1.get(char) + 1);        } else {            d1.set(char, 1);        }    }     // Update value of each key such that    // if frequency of a key is equal to 1, then it is set to 0,    // else set the value equal to null    d1.forEach(function(value, key) {        if (value === 1) {            d1.set(key, 0);        } else {            d1.set(key, null);        }    });     return d1;} function firstNotRepeatingCharacter(s) {    let d = makeHashMap(s);     // variable to store the first    // non repeating character.    let nonRep = null;     // Traversing through the string only once.    for (let i = 0; i < s.length; i++) {        let char = s.charAt(i);        if (d.get(char) !== null) {            /*            As soon as the first character in the string is            found whose value in the HashMap is "not null",            that is our first non-repeating character.            So we store it in nonRep and break out of the            loop. Thus saving on time.            */            nonRep = char;            break;        }    }     if (nonRep !== null) {        return nonRep;    } else {        // If there are no non-repeating characters        // then "_" is returned        return "_";    }} // Driver codeconsole.log(firstNotRepeatingCharacter("bbcdcca")); // This code is contributed by Prajwal Kandekar

Output

d

Time Complexity: O(N)
Auxiliary Space: O(N)

#### Method 3:  Another simple approach to this problem without using any hashmap or array is mentioned below. We can find the first non-repeating character by just using single for loop.

Another Approach:

To count the frequency of character we can do the following step:

frequency of a character = length_of_string – length_of_string_without_that _character

for example: Given String is “helloh” and we want to count frequency of character “h” so by using the above formula we can say

frequency of character “h” = 6(length of string) – 4(length of string without “h”)  = 2

So by this way, we can count the frequency of every character in a string and then if we found count == 1 that means that character is the first non-repeating character in the string.

Implementation: Implementation of the above method in java is shown below:

## C++

 #includeusing namespace std; // C++ implementation of above approach void firstNonRepeating(string s){    bool flag = false;    int index = -1;    for (int i = 0; i < s.size(); i++) {         // Here I am replacing a character with "" and        // then finding the length after replacing and        // then subtracting  length of that replaced        // string from the total length of the original        // string        string temp = s;        temp.erase(remove(temp.begin(), temp.end(), s[i]), temp.end());                  int count_occurrence = s.size() - temp.size();         if (count_occurrence == 1)        {            flag = true;            index = i;            break;        }    }     if (flag)        cout << "First non repeating character is " << s[index] << endl;     else        cout << "There is no non-repeating character is present in the string" << endl;}   int main(){         // Driver Code    string s = "GeeksforGeeks";    firstNonRepeating(s);         return 0;} // The code is contributed by Nidhi goel.

## Java

 // Java program for the above approachpublic class first_non_repeating {          static void firstNonRepeating(String s)    {        boolean flag = false;         int index = -1;         for (int i = 0; i < s.length(); i++) {             // Here I am replacing a character with "" and            // then finding the length after replacing and            // then subtracting  length of that replaced            // string from the total length of the original            // string            int count_occurrence                = s.length()                  - s.replace(                         Character.toString(s.charAt(i)),                         "")                        .length();             if (count_occurrence == 1) {                 flag = true;                 index = i;                 break;            }        }         if (flag)            System.out.println(                "First non repeating character is "                + s.charAt(index));         else            System.out.println(                "There is no non-repeating character is present in the string");    }     // Driver Code    public static void main(String arg[])    {         String s = "GeeksforGeeks";         firstNonRepeating(s);    }}// This Solution is contributed by Sourabh Sharma.

## Python3

 # Python implementation of above approachdef firstNonRepeating(s):     flag = False    index = -1    for i in range(len(s)):         # Here I am replacing a character with "" and        # then finding the length after replacing and        # then subtracting  length of that replaced        # string from the total length of the original        # string        count_occurrence = len(s) - len(s.replace(s[i],''))         if (count_occurrence == 1):            flag = True            index = i            break     if (flag):        print(f"First non repeating character is {s[index]}")     else:        print("There is no non-repeating character is present in the string") # Driver Codes = "GeeksforGeeks"firstNonRepeating(s) # This code is contributed by shinjanpatra

## C#

 // C# program for the above approachusing System; class first_non_repeating {     static void firstNonRepeating(string s)    {        bool flag = false;         int index = -1;         for (int i = 0; i < s.Length; i++) {             // Here I am replacing a character with "" and            // then finding the length after replacing and            // then subtracting  length of that replaced            // string from the total length of the original            // string            int count_occurrence                = s.Length                  - s.Replace(Char.ToString(s[i]), "")                        .Length;             if (count_occurrence == 1) {                 flag = true;                 index = i;                 break;            }        }         if (flag) {            Console.WriteLine(                "First non repeating character is "                + s[index]);        }         else {            Console.WriteLine(                "There is no non-repeating character is present in the string");        }    }     // Driver Code    public static void Main()    {         string s = "GeeksforGeeks";         firstNonRepeating(s);    }}// This Solution is contributed by Samim Hossain Mondal.

## Javascript



Output

First non repeating character is f

Time Complexity: O(N)
Auxiliary Space: O(1)