public int firstMissingPositive(int[] nums) {

for (int i = 0; i < nums.length; i++) {

while (nums[i] - 1 >= 0 && nums[i] - 1 < nums.length && nums[nums[i] - 1] != nums[i]) {

swap(nums, i, nums[i] - 1);

}

}

for (int i = 0; i < nums.length; i++) {

if (i != nums[i] - 1) {

return i + 1;

}

}

return nums.length + 1;

}

private void swap(int[] nums, int i, int j) {

if (i != j) {

int t = nums[i];

nums[i] = nums[j];

nums[j] = t;

}

}