[LeetCode] Combination Sum III

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]]

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

解题思路：

题意是，从1-9这9个数中，选出k个不同的数，使之和等于指定的值n。

理解题意后，题目就比较容易了。用一个递归模拟回溯即可。注意vector有个pop_back()的方法。这非常不错。

class Solution {
public:
    vector<vector<int>> combinationSum3(int k, int n) {
        vector<vector<int>> result;
        
        vector<int> item;
        helper(result, item, 0, k, n);
        
        return result;
    }
    
    //max是item中最大的那个数，left是还剩下的值，k是需要找的数目
    void helper(vector<vector<int>>& result, vector<int>& item, int max, int k, int left){
        if(item.size()==k&&left==0){
            result.push_back(item);
            return;
        }
        for(int i=max+1; i<=9&&i<=left; i++){
            item.push_back(i);
            helper(result, item, i, k, left-i);
            item.pop_back();
        }
    }
};