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First Missing Positive

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

var swap = function (arr, i, j) {
var tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
};

Complexity:

  • Time complexity: O(n).
  • Space complexity: O(1).