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
本質上是桶排序 (bucket sort)，每當 nums[i]!= i+1 的時候，將 nums[i] 與 nums[nums[i]-1] 交換，直到無法 交換爲止，終止條件是 nums[i]== nums[nums[i]-1]。

代碼

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]);
            }
        }
    }
};