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[LeetCode 41] First Missing Positive

Problem

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.

Example 1:

Input: nums = [1,2,0]
Output: 3

Example 2:

Input: nums = [3,4,-1,1]
Output: 2

Example 3:

Input: nums = [7,8,9,11,12]
Output: 1

Constraints:

- 1 <= nums.length <= 5 * 105
- -231 <= nums[i] <= 231 - 1

Javascript

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

	// update all negative numbers to 0, since they will not affect the final result
	for(let i = 0; i < nums.length; i++){
		if(nums[i] < 0) {
			nums[i] = 0
		}
	}

    // we will mark index i as negative, indicating i + 1 exists in array, thus using the array as Set
	for(let i = 0; i < nums.length; i++) {
		let val = Math.abs(nums[i])
		if(1 <= val && val <= nums.length) {
			if(nums[val - 1] > 0) {
				nums[val - 1] *= -1
			} else if(nums[val - 1] === 0) {
				nums[val - 1] = -1 * (nums.length + 1)
			}
		}
	}

    // in solution set, we loop from the smallest possible value and check for positive or 0 values
	for(let i = 1; i < nums.length + 1; i++) {
		if(nums[i - 1] >= 0) {
			return i
		}
	}

	return nums.length + 1
};

Reference