Given two integers n and k, return all possible combinations of knumbers out of 1 ... n.
Example:
Input: n = 4, k = 2
Output:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]
这是一道常规的dfs题,之前也对相应的题进行过训练,可惜好几天不刷,动手的时候又不知道该怎么写了。由于没什么特殊性,且题目和46题全排列基本类似,这里直接上代码,记录一下。
class Solution {
public:
vector<vector<int>> combine(int n, int k) {
vector<vector<int>> ret;
vector<int> tmp;
dfsCombine(ret, tmp, n, k, 1);
return ret;
}
void dfsCombine(vector<vector<int>> &ret, vector<int> &tmp, int n, int k, int start)
{
if(tmp.size() == k)
{
ret.push_back(tmp);
return;
}
for(int i = start; i <= n; i++)
{
tmp.push_back(i);
dfsCombine(ret, tmp, n, k, i+1);
tmp.pop_back();
}
}
};