Find the last non repeating character in string

Last Updated : 15 Mar, 2023

Given a string str, the task is to find the last non-repeating character in it.
For example, if the input string is "GeeksForGeeks", then the output should be 'r' and if the input string is "GeeksQuiz" then the output should be 'z'. if there is no non-repeating character then print -1.
Examples:

Input: str = "GeeksForGeeks"
Output: r
'r' is the first character from the end which has frequency 1.
Input: str = "aabbcc"
Output: -1
All the characters of the given string have frequencies greater than 1.

Approach: Create a frequency array that will store the frequency of each of the characters of the given string. Once the frequencies have been updated, start traversing the string from the end character by character and for every character, if the frequency of the current character is 1 then this is the last non-repeating character. If all the characters have a frequency greater than 1 then print -1.
Below is the implementation of the above approach:

C++

// C++ implementation of the approach #include <bits/stdc++.h>  using namespace std;  // Maximum distinct characters possible const int MAX = 256;  // Function to return the last non-repeating character static string lastNonRepeating(string str, int n) {      // To store the frequency of each of     // the character of the given string     int freq[MAX] = {0};      // Update the frequencies     for (int i = 0; i < n; i++)         freq[str.at(i)]++;           // Starting from the last character     for (int i = n - 1; i >= 0; i--)      {          // Current character         char ch = str.at(i);          // If frequency of the current character is 1         // then return the character         if (freq[ch] == 1)         {             string res;             res+=ch;             return res;         }     }      // All the characters of the     // string are repeating     return "-1"; }  // Driver code int main()  {     string str = "GeeksForGeeks";     int n = str.size();     cout<< lastNonRepeating(str, n);     return 0; }  // This code has been contributed by 29AjayKumar

Java

// Java implementation of the approach public class GFG {      // Maximum distinct characters possible     static final int MAX = 256;      // Function to return the last non-repeating character     static String lastNonRepeating(String str, int n)     {          // To store the frequency of each of         // the character of the given string         int freq[] = new int[MAX];          // Update the frequencies         for (int i = 0; i < n; i++)             freq[str.charAt(i)]++;          // Starting from the last character         for (int i = n - 1; i >= 0; i--) {              // Current character             char ch = str.charAt(i);              // If frequency of the current character is 1             // then return the character             if (freq[ch] == 1)                 return ("" + ch);         }          // All the characters of the         // string are repeating         return "-1";     }      // Driver code     public static void main(String[] args)     {         String str = "GeeksForGeeks";         int n = str.length();         System.out.println(lastNonRepeating(str, n));     } }

Python3

# Python3 implementation of the approach   # Maximum distinct characters possible  MAX = 256;   # Function to return the last non-repeating character  def lastNonRepeating(string, n) :       # To store the frequency of each of      # the character of the given string      freq = [0]*MAX;       # Update the frequencies      for i in range(n) :         freq[ord(string[i])] += 1;       # Starting from the last character      for i in range(n-1,-1,-1) :          # Current character          ch = string[i];           # If frequency of the current character is 1          # then return the character          if (freq[ord(ch)] == 1) :             return ("" + ch);            # All the characters of the      # string are repeating      return "-1";    # Driver code  if __name__ == "__main__" :           string = "GeeksForGeeks";      n = len(string);           print(lastNonRepeating(string, n));   # This code is contributed by AnkitRai01

// C# implementation of the approach using System;      class GFG  {      // Maximum distinct characters possible     static readonly int MAX = 256;      // Function to return the last non-repeating character     static String lastNonRepeating(String str, int n)     {          // To store the frequency of each of         // the character of the given string         int []freq = new int[MAX];          // Update the frequencies         for (int i = 0; i < n; i++)             freq[str[i]]++;          // Starting from the last character         for (int i = n - 1; i >= 0; i--)          {              // Current character             char ch = str[i];              // If frequency of the current character is 1             // then return the character             if (freq[ch] == 1)                 return ("" + ch);         }          // All the characters of the         // string are repeating         return "-1";     }      // Driver code     public static void Main(String[] args)     {         String str = "GeeksForGeeks";         int n = str.Length;         Console.WriteLine(lastNonRepeating(str, n));     } }  /* This code contributed by PrinciRaj1992 */

JavaScript

function lastNonRepeating(str, n) {     // Maximum distinct characters possible     const MAX = 256;      // To store the frequency of each of     // the character of the given string     const freq = new Array(MAX).fill(0);      // Update the frequencies     for (let i = 0; i < n; i++)         freq[str.charCodeAt(i)]++;      // Starting from the last character     for (let i = n - 1; i >= 0; i--) {         // Current character         const ch = str.charAt(i);          // If frequency of the current character is 1         // then return the character         if (freq[ch.charCodeAt()] == 1) return ch;     }      // All the characters of the     // string are repeating     return "-1"; }  const str = "GeeksForGeeks"; const n = str.length; console.log(lastNonRepeating(str, n));

Output:

Time Complexity: O(n), where n is the length of the given string.
Auxiliary Space: O(256) ? O(1), no extra space is required, so it is a constant.

K'th Non-repeating Character

29AjayKumar

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Find the last non repeating character in string

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