CodeSignal-Arrays(2):firstNotRepeatingCharacter

Question

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

• For s = "abacabad", the output should be
  solution(s) = 'c'.

  There are 2 non-repeating characters in the string: 'c' and 'd'. Return c since it appears in the string first.

• For s = "abacabaabacaba", the output should be
  solution(s) = '_'.

  There are no characters in this string that do not repeat.

Input/Output

• [execution time limit] 3 seconds (java)

• [memory limit] 1 GB

• [input] string s

  A string that contains only lowercase English letters.

  Guaranteed constraints:
  1 ≤ s.length ≤ 105.

• [output] char

  The first non-repeating character in s, or '_' if there are no characters that do not repeat.

Answer java

char solution(String str) {
if(str==null)
            return '_';
        char[] chars=str.toCharArray();
        LinkedHashMap hashMap=new LinkedHashMap();
        for(char item:chars){
            if(hashMap.containsKey(item))
                hashMap.put(item, hashMap.get(item)+1);
            else
                hashMap.put(item, 1);
            
        }
        for(char key:hashMap.keySet()){
            if(hashMap.get(key)==1)
                return key;
        }
        return '_';

}