Leetcode 216. Combination Sum III

216. Combination Sum III

• Total Accepted: 38824
• Total Submissions: 102089
• Difficulty: Medium

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.

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]]

思路：回溯法，基本思路和类似，但要注意每个组合中不能有重复元素，例如k=3，n=7，那么[1,1,5]就不是答案。

代码：

class Solution {
public:
    vector<vector<int> > res;
    void combinationSum3(vector<int> v,int k,int n,int cur){
        if(!n&&!k){
            res.push_back(v);
            return;
        }
        if(k<=0){
            return;
        }
        int i;
        for(i=cur;i<10;i++){
            n-=i;
            if(n>=0){
                v.push_back(i);
                combinationSum3(v,k-1,n,i+1);
                v.pop_back();
            }
            else{
                return;
            }
            n+=i;
        }
    }
    vector<vector<int> > combinationSum3(int k, int n) {
        vector<int> v;
        combinationSum3(v,k,n,1);
        return res;
    }
};