First Missing Positive

Description

Given an unsorted integer array, find the smallest missing positive integer.

Example 1:

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

Example 2:

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

Example 3:

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

Note:

Your algorithm should run in O(n) time and uses constant extra space.

Solution(javascript)

/**
 * @param {number[]} nums
 * @return {number}
 */
var firstMissingPositive = function(nums) {
    const len = nums.length;
    
    let middleman;
    for(let i = 0; i < len; i++){
        while(nums[i] > 0 && nums[i] <= len && (nums[nums[i]-1] != nums[i])){
            middleman = nums[nums[i]-1];
            nums[nums[i]-1] = nums[i];
            nums[i] = middleman;
        }
    }
    
    for(let i = 0; i < len; i++){
        if(nums[i] != i+1){
            return i + 1;
        }
    }
    
    return len + 1;
};