Problem

Given a string, find the first non-repeating character in it and return it's index. If it doesn't exist, return -1.
Examples:
s = "leetcode"
return 0.
s = "loveleetcode"
return 2.

Solution

// Solution:
// The optimal solution is to use a hash table to store the character and its index.
// Then iterate through the string and check if the character is in the hash table.
// If it is, then we know it is a repeating character. If it is not, then we know it is a non-repeating character.
// We can then return the index of the first non-repeating character.
// This solution is optimal because it is O(n) time complexity and O(n) space complexity.

function firstUniqChar(s) {
  let hash = {};
  for (let i = 0; i < s.length; i++) {
    if (hash[s[i]]) {
      hash[s[i]] = -1;
    } else {
      hash[s[i]] = i;
    }
  }
  for (let i = 0; i < s.length; i++) {
    if (hash[s[i]] !== -1) {
      return hash[s[i]];
    }
  }
  return -1;
} console.log(firstUniqChar("leetcode")); // 0
console.log(firstUniqChar("loveleetcode")); // 2