Given an unsorted integer array, find the smallest missing positive integer.

Example 1:

Input: [1,2,0]
Output: 3

Example 2:

Input: [3,4,-1,1]
Output: 2

Example 3:

Input: [7,8,9,11,12]
Output: 1

Note:

Your algorithm should run in O(n) time and uses constant extra space.

這個題是查詢第一個最小的正整數。

public class Solution {
    public int firstMissingPositive(int[] nums) {
        if (nums == null || nums.length == 0)  // 防止越界
            return 1;
        for (int i = 0; i < nums.length; i++) {
            //    nums[i] - 1 < nums.length 超出範圍不交換    nums[i] != nums[nums[i] - 1] 相等不交換
            while (nums[i] > 0 && nums[i] != i + 1 && nums[i] - 1 < nums.length && nums[i] != nums[nums[i] - 1]) {
                swap(nums, i, nums[i] - 1);
            }
        }
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] != i + 1) {
                return i + 1;  // 第一個不相等就返回
            }
        }
        return nums[nums.length - 1] + 1;  // 陣列交換後是有序正數,就返回最大 + 1
    }
 
    public void swap(int[] nums, int i, int j) {
        nums[i] = nums[i] ^ nums[j];
        nums[j] = nums[i] ^ nums[j];
        nums[i] = nums[i] ^ nums[j];
    }
}