Description:
- Difficulty:Medium
- Total Accepted:132.8K
- Total Submissions:327.1K
- Contributor:LeetCode
Given two integers n and k, return all possible combinations of k numbers out of 1 … n.
For example,
If n = 4 and k = 2, a solution is:
[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ]
Thought:
一开始想要用之前写的Subsets的代码,求出{1,2,…,n}集合的子集,然后取其中长度为k的集合作为result。结果是没问题的,但是测试显示超出内存了,所以行不通。所以还是需要写算法出来。一个个尝试数字的排列,并选取长度为k的加入到内存中。
Code:
class Solution {
public:
//eg 4,1 [1][2][3][4]
//eg 4,2 len = 6 [1,2][1,3][1,4][2,3][2,4][3,4]
//eg 4,3 [1,2,3] [1,2,4] [1,3,4] [2,3,4]
vector<vector<int>> combine(int n, int k) {
if (n < 0 || n < k) return vector<vector<int>>();
vector<vector<int>> result;
vector<int> item;
helper(n, k, 1, item, result);
return result;
}
void helper(int n, int k, int start, vector<int>& nums, vector<vector<int>>& result)
{
if (nums.size() == k) {
result.push_back(nums);
return;
}
for (int i = start; i <= n; i++)
{
nums.push_back(i);
helper(n, k, i + 1, nums, result);
nums.pop_back();
}
}
};