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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:

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#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