First Unique Character in a String

Problem

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

 

Constraints:

• 1 <= s.length <= 105
• s consists of only lowercase English letters.

Solution

/**
 * @param {string} s
 * @return {number}
 */
var firstUniqChar = function(s) {
    let map = {};
    for (let c of s) {
        if (c in map) {
            map[c] = true;
        } else {
            map[c] = false;
        }
    }

    for (let i = 0; i < s.length; ++i) {
        if (!map[s[i]]) {
            return i;
        }
    }
    return - 1;
};

Iterate through s and keep track of if a character has appeared before using map (true if a character has appeared multiple times and false if it appeared once). Next, iterate through s again and return the index of the first character where its entry in map is false.