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First non-repeating character of given string

Last Updated : 07 Apr, 2025
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Given a string s of lowercase English letters, the task is to find the first non-repeating character. If there is no such character, return '$'.

Examples: 

Input: s = "geeksforgeeks"
Output: 'f'
Explanation: 'f' is the first character in the string which does not repeat.

Input: s = "racecar"
Output: 'e'
Explanation: 'e' is the only character in the string which does not repeat.

Input: "aabbccc"
Output: '$'
Explanation: All the characters in the given string are repeating.

Table of Content

[Naive Approach] Using Nested Loop – O(n^2) Time and O(1) Space

The idea is to use two nested loops, the outer loop for picking an element and the inner loop for finding another occurrence of the picked character in the string. As soon as we find a character which has only one occurrence in the input string, return it. If all characters have multiple occurrences, return '$'.

C++ 
#include <bits/stdc++.h>
using namespace std;

char nonRep(string &s) {
    int n = s.length();
    for (int i = 0; i < n; ++i) {
        bool found = false;

        for (int j = 0; j < n; ++j) {
            if (i != j && s[i] == s[j]) {
                found = true;
                break;
            }
        }
        if (!found) 
            return s[i];
    }

    return '$';
}

int main() {
    string s = "racecar";
    cout << nonRep(s);
    return 0;
}
Java 
public class GFG {

    public static char nonRep(String s) {
        int n = s.length();
        for (int i = 0; i < n; ++i) {
            boolean found = false;

            for (int j = 0; j < n; ++j) {
                if (i != j && s.charAt(i) == s.charAt(j)) {
                    found = true;
                    break;
                }
            }
            if (!found) 
                return s.charAt(i);
        }

        return '$';
    }

    public static void main(String[] args) {
        String s = "racecar";
        System.out.println(nonRep(s));
    }
}
Python 
def nonRep(s):
    n = len(s)
    for i in range(n):
        found = False
        for j in range(n):
            if i != j and s[i] == s[j]:
                found = True
                break
        if not found:
            return s[i]
    return '$'


s = "racecar"
print(nonRep(s))
C# 
using System;

class GFG {

    public static char nonRep(string s) {
        int n = s.Length;
        for (int i = 0; i < n; ++i) {
            bool found = false;

            for (int j = 0; j < n; ++j) {
                if (i != j && s[i] == s[j]) {
                    found = true;
                    break;
                }
            }
            if (!found)
                return s[i];
        }

        return '$';
    }

    static void Main(string[] args) {
        string s = "racecar";
        Console.WriteLine(nonRep(s));
    }
}
JavaScript 
function nonRep(s) {
    let n = s.length;
    for (let i = 0; i < n; ++i) {
        let found = false;
        for (let j = 0; j < n; ++j) {
            if (i !== j && s[i] === s[j]) {
                found = true;
                break;
            }
        }
        if (!found) return s[i];
    }
    return '$';
}

let s = "racecar";
console.log(nonRep(s));

Output
e

[Efficient Approach 1] Using Frequency Array (Two Traversal) – O(2*n) Time and O(MAX_CHAR ) Space

The efficient approach is to either use an array of size MAX_CHAR  to store the frequencies of each character. Traverse the input string twice:

  • First traversal is to count the frequency of each character.
  • Second traversal to find the first character in string with a frequency of one.
C++ 
#include <bits/stdc++.h>
using namespace std;
const int MAX_CHAR = 26;  

char nonRep(const string &s) {
    vector<int> freq(MAX_CHAR, 0);
    for (char c : s) {
        freq[c - 'a']++;
    }

    // Find the first character with frequency 1
    for (char c : s) {
        if (freq[c - 'a'] == 1) {
            return c;
        }
    }
    return '$';
}

int main() {
    string s = "geeksforgeeks";
    cout << nonRep(s) << endl;
    return 0;
}
Java 
import java.util.*;

public class GFG {
    private static final int MAX_CHAR = 26;  

    public static char nonRep(String s) {
        int[] freq = new int[MAX_CHAR];

        for (char c : s.toCharArray()) {
            freq[c - 'a']++;
        }

        // Find the first character with frequency 1
        for (char c : s.toCharArray()) {
            if (freq[c - 'a'] == 1) {
                return c;
            }
        }
        return '$';
    }

    public static void main(String[] args) {
        String s = "geeksforgeeks";
        System.out.println(nonRep(s));
    }
}
Python 
MAX_CHAR = 26  

def nonRep(s):
    freq = [0] * MAX_CHAR

    for c in s:
        freq[ord(c) - ord('a')] += 1

    # Find the first character with frequency 1
    for c in s:
        if freq[ord(c) - ord('a')] == 1:
            return c

    return '$'

s = "geeksforgeeks"
print(nonRep(s))
C# 
using System;

class GFG {
    const int MAX_CHAR = 26;  

    static char nonRep(string s) {
        int[] freq = new int[MAX_CHAR];

        foreach (char c in s) {
            freq[c - 'a']++;
        }

        // Find the first character with frequency 1
        foreach (char c in s) {
            if (freq[c - 'a'] == 1) {
                return c;
            }
        }
        return '$';
    }

    static void Main() {
        string s = "geeksforgeeks";
        Console.WriteLine(nonRep(s));
    }
}
JavaScript 
const MAX_CHAR = 26;  

function nonRep(s) {
    const freq = new Array(MAX_CHAR).fill(0);
    for (let c of s) {
        freq[c.charCodeAt(0) - 'a'.charCodeAt(0)]++;
    }

    // Find the first character with frequency 1
    for (let c of s) {
        if (freq[c.charCodeAt(0) - 'a'.charCodeAt(0)] === 1) {
            return c;
        }
    }
    return '$';
}

let s = "geeksforgeeks";
console.log(nonRep(s));

Output
f

[Efficient Approach 2] By Storing Indices (Single Traversal) – O(n) Time and O(MAX_CHAR ) Space

The above approach can be optimized using a single traversal of the string. The idea is to maintain a visited array of size MAX_CHAR initialized to -1, indicating no characters have been seen yet. Now, we iterate through the string:

  • If a character is seen for the first time, its index is stored in the array.
  • If the character is found again then its array value is set to -2 to represent this character is now repeating.

After the string traversal, traverse the visited array and check if value in the array is not equal to -1 or -2 (means, this character is not repeating). We then find the smallest positive index from these values to find the first non-repeating character. If no such index is found, return '$'.


C++ 
#include <bits/stdc++.h>
using namespace std;

const int MAX_CHAR = 26;  

char nonRep(const string& s) {
    vector<int> vis(MAX_CHAR, -1);
    for (int i = 0; i < s.length(); ++i) {
        int index = s[i] - 'a';
        if (vis[index] == -1) {
            
            // Store the index when character is first seen
            vis[index] = i;  
        } else {
            
            // Mark character as repeated
            vis[index] = -2; 
        }
    }

    int idx = -1;

    // Find the smallest index of the non-repeating characters
    for (int i = 0; i < MAX_CHAR; ++i) {
        if (vis[i] >= 0 && (idx == -1 || vis[i] < vis[idx])) {
            idx = i;
        }
    }
    return (idx == -1) ? '$' : s[vis[idx]];
}

int main() {
    string s = "aabbccc";
    cout << nonRep(s) << endl;
    return 0;
}
Java 
import java.util.*;

public class Main {
    static final int MAX_CHAR = 26;

    public static char nonRep(String s) {
        int[] vis = new int[MAX_CHAR];
        Arrays.fill(vis, -1);

        for (int i = 0; i < s.length(); i++) {
            int index = s.charAt(i) - 'a';
            if (vis[index] == -1) {
                
                  // Store the index when character is first seen
                vis[index] = i;
            } else {
                
                // Mark character as repeated
                vis[index] = -2;  
            }
        }

        int idx = -1;

        // Find the smallest index of the non-repeating characters
        for (int i = 0; i < MAX_CHAR; i++) {
            if (vis[i] >= 0 && (idx == -1 || vis[i] < vis[idx])) {
                idx = i;
            }
        }

        return (idx == -1) ? '$' : s.charAt(vis[idx]);
    }

    public static void main(String[] args) {
        String s = "aabbccc";
        System.out.println(nonRep(s));
    }
}
Python 
def nonRep(s):
    MAX_CHAR = 26
    vis = [-1] * MAX_CHAR

    for i in range(len(s)):
        index = ord(s[i]) - ord('a')
        if vis[index] == -1:
            
            # Store the index when character is first seen
            vis[index] = i  
        else:
            
            # Mark character as repeated
            vis[index] = -2  

    idx = -1

    # Find the smallest index of the non-repeating characters
    for i in range(MAX_CHAR):
        if vis[i] >= 0 and (idx == -1 or vis[i] < vis[idx]):
            idx = i

    return '$' if idx == -1 else s[vis[idx]]


s = "aabbccc"
print(nonRep(s))
C# 
using System;
class GFG
{
    const int MAX_CHAR = 26;

    static char nonRep(string s)
    {
        int[] vis = new int[MAX_CHAR];
        Array.Fill(vis, -1);

        for (int i = 0; i < s.Length; i++)
        {
            int index = s[i] - 'a';
            if (vis[index] == -1)
            {
                // Store the index when character is first seen
                vis[index] = i;  
            }
            else
            {
                // Mark character as repeated
                vis[index] = -2;  
            }
        }

        int idx = -1;

        // Find the smallest index of the non-repeating characters
        for (int i = 0; i < MAX_CHAR; i++)
        {
            if (vis[i] >= 0 && (idx == -1 || vis[i] < vis[idx]))
            {
                idx = i;
            }
        }

        return idx == -1 ? '$' : s[vis[idx]];
    }

    static void Main()
    {
        string s = "aabbccc";
        Console.WriteLine(nonRep(s));
    }
}
JavaScript 
function nonRep(s) {
    const MAX_CHAR = 26;
    let vis = new Array(MAX_CHAR).fill(-1);

    for (let i = 0; i < s.length; i++) {
        let index = s.charCodeAt(i) - 'a'.charCodeAt(0);
        if (vis[index] === -1) {
            // Store the index when character is first seen
            vis[index] = i;  
        } else {
            // Mark character as repeated
            vis[index] = -2; 
        }
    }

    let idx = -1;

    // Find the smallest index of the non-repeating characters
    for (let i = 0; i < MAX_CHAR; i++) {
        if (vis[i] >= 0 && (idx === -1 || vis[i] < vis[idx])) {
            idx = i;
        }
    }

    return idx === -1 ? '$' : s[vis[idx]];
}

let s = "aabbccc";
console.log(nonRep(s));

Output
$

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