leetcode：41. First Missing Positive (Java)

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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.

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

代码实现（Java）：

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