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();
}
}
}
};