387. First Unique Character in a String

Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1.

Example 1:

Input: s = "leetcode"
Output: 0

Example 2:

Input: s = "loveleetcode"
Output: 2

Example 3:

Input: s = "aabb"
Output: -1 

Solution:

class Solution {
    public int firstUniqChar(String s) {
        Map<Character, Integer> map = new HashMap<Character, Integer>();
        int result = -1;
        for (int i = 0; i < s.length(); i++){
            map.put(s.charAt(i), map.getOrDefault(s.charAt(i), 0) + 1);
        }

        for (int i = 0; i < s.length(); i++){
            if (map.get(s.charAt(i)) == 1){
                result = i;
                break;
            }
        }
        return result;
    }
}


// TC: O(n)
// Sc: O(1) // because English alphabet contains 26 letters

class Solution {
    public int firstUniqChar(String s) {

        Map<Character, Integer> map = new HashMap<>();
        int cur = 0;
        for (int i = 0; i < s.length(); i++){
            map.put(s.charAt(i), map.getOrDefault(s.charAt(i), 0) + 1);

        }

        for (int i = 0; i < s.length(); i++){
            if (map.get(s.charAt(i)) == 1){
                return i;
            }
        }

        return -1;

    }
}

// TC: O(n)
// SC: O(n)