First Unique Character in a String

Step 1:

First you want to make a map with the key as the character and index as number of time the character occurs.

var charCount = new Map();
for(var i = 0; i < s.length; i++){
    var ch = s[i]; // get character
    //check if char exists in map & inc count on occurrence
    if(charCount[ch]){ 
       charCount[ch]++;
    }else charCount[ch]= 1 
}

Step 2 :

Then, loop through the string again and reference the “charCount” map to check if it is a repeating ( > 1) or non repeating (1) occurrence. The first character to have 1 as the char count value is the “First Unique Character”

for(var i = 0; i < s.length; i++){
    //find the first letter with a charCount of 1 (non repeating)
    var ch = s[i];
    if(charCount[ch] == 1){
        return i;
    }
};
return -1;

Return index if character if found. Return -1 if not found or empty string.

Full code:

var firstUniqChar = function(s) {
    
  var charCount = new Map();
  for(var i = 0; i < s.length; i++){
      var ch = s[i];
      if(charCount[ch]){
        charCount[ch]++
      }else charCount[ch]= 1
  }
  for(var i = 0; i < s.length; i++){
      //find the first letter with a charCount of 1 (non repeating)
      var ch = s[i];
      if(charCount[ch] == 1){
          return i;
      }
  };
  return -1; 
    
};

Time complexity -> O(n)

Space complexity -> O(n)

Problem: https://leetcode.com/problems/first-unique-character-in-a-string/