First Missing Positive

Given an unsorted integer array, find the first missing positive integer. For example, Given [1,2,0] return 3, and [3,4,-1,1] return 2.

• Time: O(n)
• Space: O(1)

public int firstMissingPositive(int[] nums) {
    for (int i = 0; i < nums.length; i++) {
        while (nums[i] > 0 && 
               nums[i] < nums.length && 
               nums[i] != nums[nums[i] - 1]) {
            swap(nums, i, nums[i] - 1);
        }
    }
    for (int i = 0; i < nums.length; i++) {
        if (i + 1 != nums[i]) {
            return i + 1;
        }
    }
    return nums.length + 1;
}

private void swap(int[] nums, int i, int j) {
    int tmp = nums[j];
    nums[j] = nums[i];
    nums[i] = tmp;
}