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  • Combination Sum III -- leetcode

    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:


    基本思路:

    使用dfs进行穷举遍历。

    因为结果集要求按升序排列,则先选择较小的数字,然后从剩下的较大数中进行选择。

    确保每次选择的数都大于上次选择的。


    class Solution {
    public:
        vector<vector<int>> combinationSum3(int k, int n) {
            vector<vector<int>> ans;
            vector<int> seq;
            helper(ans, seq, k, n);
            return ans;
        }
        
        void helper(vector<vector<int>>& ans, vector<int>& seq, int k, int n) {
            if (!k && !n)
                return ans.push_back(seq);
            
            if (!k || n<0)
                return;
            
            int start = seq.empty() ? 1 : seq.back()+1;
            for (int i=start; i<=9; i++) {
                seq.push_back(i);
                helper(ans, seq, k-1, n-i);
                seq.pop_back();
            }
        }
    };


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  • 原文地址:https://www.cnblogs.com/gccbuaa/p/7116037.html
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