[Array]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.

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]]

方法：递归调用即可

class Solution {private:      void recursive(int sum,vector<int>& combination,vector<vector<int>>& res,int now,int target,int layer,int k){          if(sum==target&&layer==k){              res.push_back(combination);              return;          }          for(int i=now+1;i<=9;i++){              if(layer+1>k)                return;              combination.push_back(i);              recursive(sum+i,combination,res,i,target,layer+1,k);              combination.pop_back();          }      }public:    vector<vector<int>> combinationSum3(int k, int n) {        vector<vector<int>> res;        vector<int> combination;        recursive(0,combination,res,0,n,0,k);        return res;    }};