First Missing Positive

function firstMissingPositive(nums) {
  if (nums.length === 0) return 1;

  let n = nums.length;

  for (let i = 0; i < n; i++) {
    while (nums[i] >= 1 && nums[i] <= n && nums[nums[i] - 1] !== nums[i]) {
      let temp = nums[nums[i] - 1];

      nums[nums[i] - 1] = nums[i];

      nums[i] = temp;
    }
  }

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

  return n + 1;
}