【leetcode】【medium】216. Combination Sum III

216. 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.

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]]

题目链接：https://leetcode-cn.com/problems/combination-sum-iii/

思路

法一：回溯法

和其他回溯法的题目做法类似，向下传递结果。

剪枝：因为是组合，且每个数只能用一次，则下一层迭代从上次取的数字之后开始。

终止条件是k==0（也可以是k==1）。

小坑：组合中的数字在[1,9]范围内。

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class Solution {
public:
    vector<vector<int>> res;
    vector<vector<int>> combinationSum3(int k, int n) {
        if(k<=0 || n<=0 || n<k) return res;
        vector<int> cur;
        trace(k, n, 0, cur);
        return res;
    }
    void trace(int k, int n, int last, vector<int> &cur){
        if(k==0){
            if(n==0) res.push_back(cur);
            return;
        }
        for(int i = last+1; i<=9; ++i){
            if(i>n) break;
            cur.push_back(i);
            trace(k-1, n-i, i, cur);
            cur.pop_back();
        }
        return;
    }
};