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Find First Non-Repeating Character in String

Given a string s, find the first character that does not repeat. If no such character exists, return -1.

Difficulty:

JavaScript Solution

function firstUniqueChar(s) {
  const freq = {};
  for (let ch of s) freq[ch] = (freq[ch] || 0) + 1;
  for (let i = 0; i < s.length; i++) {
    if (freq[s[i]] === 1) return i;
  }
  return -1;
}

Explanation: We count character frequency and then find the first one with count 1.

⏱ O(n) | 💾 O(n)

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Find First Non-Repeating Character in String