377. Combination Sum IV Medium

Given an integer array with all positive numbers and no duplicates, find the number of possible combinations that add up to a positive integer target.

思路：找到子问题为：target减少的时候一共有多少中排列方式。对于target有多少种排列方式，就是将所有可能组成的target*的排列个数相加。注意我们认为target为0时有1中个排列方式，这是用于排列数和target相等的情况。

class Solution {public:    int combinationSum4(vector<int>& nums, int target) {        int result[target + 1];        for (int i = 0; i < target + 1; i++) {            result[i] = 0;            }        result[0] = 1;        for (int i = 1; i < target + 1; i++) {            for (int j = 0; j < nums.size(); j++) {                if (nums[j] <= i) {                    result[i] += result[i - nums[j]];                }            }            }        return result[target];    }};