LeetCode 41. First Missing Positive

描述
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.

分析
本质上是桶排序 (bucket sort)，每当 nums[i]!= i+1 的时候，将 nums[i] 与 nums[nums[i]-1] 交换，直到无法 交换为止，终止条件是 nums[i]== nums[nums[i]-1]。

代码

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