题目描述
Find all valid combinations of
k
numbers that sum up ton
such that the following conditions are true:
- Only numbers
1
through9
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