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]]

思路：给定一个数k和一个数n。求k个1-9的数字相加之和等于给定的数字n。

class Solution {public:    vector<vector<int>> combinationSum3(int k, int n) {        vector<int>slo;        vector<vector<int>>res;        vector<int>nums(9);        if(k<=0||n<=0||9*k<n)            return res;        for(int i=0;i<nums.size();i++)             nums[i]=i+1;        DFS(nums,0,slo,res,n,k);        return res;    }        void DFS(vector<int>&nums,int index,vector<int>&slo,vector<vector<int>>&res,int n,int k)    {        if(n==0&&slo.size()==k)        {            res.push_back(slo);            return;        }        for(int i=index;i<nums.size();i++)        {            if(nums[i]<=n&&slo.size()<=k)            {                slo.push_back(nums[i]);                DFS(nums,++index,slo,res,n-nums[i],k);                slo.pop_back();            }        }    }};