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
• Explanation: The character 'l' at index 0 is the first character that does not occur at any other index.

Example 2:

• Input: s = "loveleetcode"
• Output: 2

Example 3:

• Input: s = "aabb"
• Output: -1

Constraints:

• 1 <= s.length <= 10^5
• s consists of only lowercase English letters.

Approach

Solution

/**
 * @param {string} s
 * @return {number}
 */
var firstUniqChar = function (s) {
  const freq = {};
  for (let c of s) freq[c] = (freq[c] || 0) + 1;

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

Explanation