41. 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.

《剑指offer》P261 面试题51
比较类似的思路，O(n)时间、O(1)空间排序，可以用这个方法。

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