code.java

class Solution {
    public int firstMissingPositive(int[] nums) {
        for (int i = 0; i < nums.length; ++i) {
            if (nums[i] == i + 1) {
                continue;
            }
            int n = nums[i];
            nums[i] = -1;
            while (n > 0 && n <= nums.length && nums[n - 1] != n) {
                int t = nums[n - 1];
                nums[n - 1] = n;
                n = t;
            }
        }
        for (int i = 0; i < nums.length; ++i) {
            if (nums[i] != i + 1) {
                return i + 1;
            }
        }
        return nums.length + 1;
    }
}