#41 - First Missing Positive

Hard • View on LeetCode • View on GitHub

Problem Description

Given an unsorted integer array nums, return the smallest missing positive integer.

You must implement an algorithm that runs in O(n) time and uses constant extra space.

Solution

/**
 * @param {number[]} nums
 * @return {number}
 */
var firstMissingPositive = function(nums) {
  let i = 0;

  while (i < nums.length) {
    if (nums[i] > 0 && nums[i] <= nums.length && nums[nums[i] - 1] !== nums[i]) {
      [nums[nums[i] - 1], nums[i]] = [nums[i], nums[nums[i] - 1]];
    } else {
      i++;
    }
  }

  for (i = 0; i < nums.length; i++) {
    if (nums[i] !== i + 1) {
      return i + 1;
    }
  }

  return i + 1;
};