LeetCode #216 - Combination Sum III

题目描述：

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.

Note:

• All numbers will be positive integers.
• The solution set must not contain duplicate combinations.

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]]

由于指定了从1到9，还是可以利用递归解决，而且不用担心元素重复的问题。

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