Day 14: First Non-Repeating Character – JavaScript DSA

🚀 Day 14 – First Non-Repeating Character in JavaScript

This string problem is frequently asked in product companies like Amazon and PayPal.

🧪 Problem:

Given a string, return the index of the first non-repeating character. If no such character exists, return -1.

📌 Examples:


Input: "leetcode"      ➡️ Output: 0  

Input: "loveleetcode"  ➡️ Output: 2  

Input: "aabbcc"         ➡️ Output: -1

✅ JavaScript Code:



function firstUniqueChar(str) {

  const freq = {};

  for (let char of str) {

    freq[char] = (freq[char] || 0) + 1;

  }

  for (let i = 0; i < str.length; i++) {

    if (freq[str[i]] === 1) return i;

  }

  return -1;

}

✨ Alternate Solution (indexOf + lastIndexOf)



function firstUniqCharBuiltIn(s) {

  for (let i = 0; i < s.length; i++) {

    if (s.indexOf(s[i]) === s.lastIndexOf(s[i])) return i;

  }

  return -1;

}

🎯 Tips:

  • Use a hash map to store character frequency
  • indexOf + lastIndexOf is slower but useful for short strings

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