First Unique Character in a String (Leetcode 387)

Leetcode 387

Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1.

Examples

• Example 1
  • Input: s = "leetcode"
  • Output: 0
• Example 2
  • Input: s = "loveleetcode"
  • Output: 2
• Example 3
  • Input: s = "aabb"
  • Output: -1

Solution 1

• Split string and loop over each character
• If indexOf and lastIndexOf are equal it means there is only 1 occurrence
  • This is the first instance of a unique character
  • Return this as the result
• If no items pass return '-1'

const firstUniqChar = (input) => {
  var inputArr = input.split("");

  for (var i = 0; i < inputArr.length; i++) {
    var char = inputArr[i];

    if (inputArr.indexOf(char) == inputArr.lastIndexOf(char)) {
      return inputArr.indexOf(char);
    }
  }

  return -1;
};

const example1 = "leetcode";
const example2 = "loveleetcode";
const example3 = "aabb";
const example4 = "dddccdbba";

console.log("example1", firstUniqChar(example1)); // 0
console.log("example2", firstUniqChar(example2)); // 2
console.log("example3", firstUniqChar(example3)); // -1
console.log("example4", firstUniqChar(example4)); // 8

Big O -> O(3n) -> O(n)

Solution 2

const firstUniqChar = (input) => {
  const resultsObj = {};
  let firstNonRepeatingCharIndex = -1;

  input.split("").forEach((char) => {
    resultsObj[char] = resultsObj[char] ? resultsObj[char] + 1 : 1;
  });

  for (const x in resultsObj) {
    if (resultsObj[x] === 1) {
      firstNonRepeatingCharIndex = input.indexOf(x);
      // break out of for loop
      break;
    }
  }

  return firstNonRepeatingCharIndex;
};

const example1 = "leetcode";
const example2 = "loveleetcode";
const example3 = "aabb";
const example4 = "dddccdbba";

console.log("example1", firstUniqChar(example1)); // 0
console.log("example2", firstUniqChar(example2)); // 2
console.log("example3", firstUniqChar(example3)); // -1
console.log("example4", firstUniqChar(example4)); // 8

Big O -> O(2n) -> O(n)