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First Unique Character in a String

Problem statement​

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.

My solution​

/**
 * @param {string} s
 * @return {number}
 */
var firstUniqChar = function(s) {
    let memo = new Map();
    for (let i = 0; i < s.length; i++) {
        const char = s.charAt(i)
        if (!memo.has(char)) {
            memo.set(char, [])
        }
        
        memo.get(char).push(i)
    }
    
    for (const list of memo.values()) {
        if (list.length === 1) {
            return list[0]
        }
    }
    
    return -1;
};