LeetCode 之 Combination Sum III

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Ensure that numbers within the set are sorted in ascending order.

Example 1:

Input: k = 3, n = 7

Output:

[[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

[[1,2,6], [1,3,5], [2,3,4]]

这道题和之前的全排列问题有点类似，先在1到9中选择一个元素i，把问题转化为（k-1,n-i）问题，这样递归解决，关键是我们要存储中间结果方便回溯，每次到达底部后（n==0和k==0）返回。代码如下：

class Solution {
public:
    vector<vector<int>> combinationSum3(int k, int n) {
        vector<vector<int>> res;
        vector<int> ans;
        int i=1;
        combinationSum3(res,ans,i,k,n);
        return res;
    }
    void combinationSum3(vector<vector<int>>& res,vector<int>& ans,int low,int k,int n){
        if(n==0&&k==0){
            res.push_back(ans);
            return;
        }else if(n>0&&k>0){
            for(int i=low;i<=n&&i<=9;i++){
                ans.push_back(i);
                combinationSum3(res,ans,i+1,k-1,n-i);
                ans.pop_back();
            }
        }
    }
};