Find First Non-Repeating Character

JavaScript

Technical Specification

Given a string, return the first character that does not repeat. If none exists, return null.

Input/Output Samples

Input:"aabbcde"

Output:"c"

Input:"aabb"

Output:null

Optimal Logic Path

function firstNonRepeatingChar(str) {
  const freq = {};
  for (const ch of str) {
    freq[ch] = (freq[ch] || 0) + 1;
  }
  for (const ch of str) {
    if (freq[ch] === 1) return ch;
  }
  return null;
}

Architectural Deep-Dive

We store counts in an object, then scan again to find the first character with frequency 1.