题目描述
题目链接:39. Combination Sum
Given an array of distinct integers
candidates
and a target integertarget
, return a list of all unique combinations ofcandidates
where the chosen numbers sum totarget
. You may return the combinations in any order.The same number may be chosen from
candidates
an unlimited number of times. Two combinations are unique if the frequency of at least one of the chosen numbers is different.It is guaranteed that the number of unique combinations that sum up to
target
is less than150
combinations for the given input.
例子
例子 1
Input:
candidates = [2,3,6,7], target = 7
Output:[[2,2,3],[7]]
Explanation: 2 and 3 are candidates, and 2 + 2 + 3 = 7. Note that 2 can be used multiple times. 7 is a candidate, and 7 = 7. These are the only two combinations.
例子 2
Input:
candidates = [2,3,5], target = 8
Output:[[2,2,2,2],[2,3,3],[3,5]]
例子 3
Input:
candidates = [2], target = 1
Output:[]
例子 4
Input:
candidates = [1], target = 1
Output:[[1]]
例子 5
Input:
candidates = [1], target = 2
Output:[[1,1]]
Constraints
1 <= candidates.length <= 30
1 <= candidates[i] <= 200
- All elements of
candidates
are distinct.1 <= target <= 500
解题思路
[Leetcode]77. Combinations(C++) 的变种,同样需要用回溯法遍历所有组合,但这道题目允许一个数字选择多次,因此在递归调用的 dfs 函数中不需要改变传入索引。并且由于数组中都为正数,因此循环截止条件可以为当前子数组和大于等于 target
。(这里我的处理方式是使用 res
表示当前子数组距 target
还有多远,因此返回条件是 res <= 0
),代码如下:
#include <vector>
class Solution {
public:
std::vector<std::vector<int>> combinationSum(std::vector<int>& candidates, int target) {
std::vector<int> candidate;
std::vector<std::vector<int>> result;
dfs(candidates, 0, target, candidate, result);
return result;
}
private:
void dfs(const std::vector<int>& candidates, int idx, int res, std::vector<int>& candidate, std::vector<std::vector<int>>& result) {
if (res == 0) {
result.push_back(candidate);
} else if (res < 0) {
return;
}
for (int i = idx; i < candidates.size(); ++i) {
candidate.push_back(candidates[i]);
dfs(candidates, i, res - candidates[i], candidate, result);
candidate.pop_back();
}
}
};
- 时间复杂度: O(2^n)
- 空间复杂度: O(2^n)
GitHub 代码同步地址: 39.CombinationSum.cpp
其他题目: GitHub: Leetcode-C++-Solution 博客: Leetcode-Solutions