41. First Missing Positive.js

var firstMissingPositive = function (nums) {
  for (let i = 0; i < nums.length; ++i) {
    while (
      nums[i] !== i + 1 &&
      nums[i] >= 1 &&
      nums[i] <= nums.length &&
      nums[nums[i] - 1] != nums[i]
    ) {
      // swap two elements
      [nums[nums[i] - 1], nums[i]] = [nums[i], nums[nums[i] - 1]];
    }
  }
  for (let i = 0; i < nums.length; ++i) {
    if (nums[i] != i + 1) {
      return i + 1;
    }
  }
  return nums.length + 1;
};