First Not Repeating Character in String - The Coding Shala

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 In this post, we will learn how to find the First Not Repeating Character in a String and will write a Java program for the same.

First Not Repeating Character in a String

Given a string s consisting of small English letters, find and return the first instance of a non-repeating character in it. If there is no such character, return '_'.

Example 1:
For s = "abacabad", the output should be firstNotRepeatingCharacter(s) = 'c'.
There are 2 non-repeating characters in the string: 'c' and 'd'. Return c since it appears in the string first.

Example 2:
For s = "abacabaabacaba", the output should be firstNotRepeatingCharacter(s) = '_'.
There are no characters in this string that do not repeat.

First Not Repeating Character in a String Java Program

Approach 1

We will use HashMap to count the occurrence of each character then will return if the count of a char is 1.

Java Program: 

char firstNotRepeatingCharacter(String s) {
char[] arr = s.toCharArray();
HashMap<Character, Integer> map = new HashMap<>();
for(int i=0;i<arr.length;i++){
    if(map.containsKey(arr[i])){
        map.put(arr[i], map.get(arr[i])+1);
    }
    else{
        map.put(arr[i], 1);
    }
}
for(int i=0;i<arr.length;i++){
    if(map.get(arr[i])==1) return arr[i];
}
return '_';
}

Approach 2

We can use String methods indexOf() and lastIndexOf(). If both the methods return the same index that means char occur only once in the string.

Java Program: 

char firstNotRepeatingCharacter(String s) {
    char[] arr = s.toCharArray();
    for(int i=0;i<arr.length;i++){
        if(s.indexOf(arr[i])==s.lastIndexOf(arr[i])) return arr[i];
    }
    return '_';
}

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