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[Leetcode]216. Combination Sum III(C++)

题目描述

题目链接:216. Combination Sum III

Find all valid combinations of k numbers that sum up to n such that the following conditions are true:

  • Only numbers 1 through 9 are used.
  • Each number is used at most once. Return a list of all possible valid combinations. The list must not contain the same combination twice, and the combinations may be returned in any order.

例子

例子 1

Input: k = 3, n = 7 Output: [[1,2,4]] Explanation: 1 + 2 + 4 = 7 There are no other valid combinations.

例子 2

Input: k = 3, n = 9 Output: [[1,2,6],[1,3,5],[2,3,4]] Explanation: 1 + 2 + 6 = 9 1 + 3 + 5 = 9 2 + 3 + 4 = 9 There are no other valid combinations.

例子 3

Input: k = 4, n = 1 Output: [] Explanation: There are no valid combinations. [1,2,1] is not valid because 1 is used twice.

Constraints

  • 2 <= k <= 9
  • 1 <= n <= 60

解题思路

参考 [Leetcode]40. Combination Sum II(C++), 思路大同小异,代码如下:

#include <vector>

class Solution {
public:
    std::vector<std::vector<int>> combinationSum3(int k, int n) {
        k_ = k;
        std::vector<int> comb;
        std::vector<std::vector<int>> result;
        dfs(comb, 1, n, result);
        return result;
    }

private:
    int k_;

    void dfs(std::vector<int>& comb, int idx, int target,
             std::vector<std::vector<int>>& result) {
        if (comb.size() == k_) {
            if (target == 0) {
                result.push_back(comb);
            }
            return;
        }
        if (target < 0) {
            return;
        }
        for (int i = idx; i <= 9; i++) {
            comb.push_back(i);
            dfs(comb, i + 1, target - i, result);
            comb.pop_back();
        }
    }
};
  • 时间复杂度: O(n!)
  • 空间复杂度: O(n!)

GitHub 代码同步地址: 216.CombinationSumIii.cpp

其他题目: GitHub: Leetcode-C++-Solution 博客: Leetcode-Solutions

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