[LeetCode] Combinations

题目描述:

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],
]

解题思路:

class Solution {
public:
vector<vector<int>> combine(int n, int k) {
vector<vector<int>> result;
vector<int> elem;

combine(n, k, 1, result, elem);
return result;
}
private:
void combine(int n, int k, int cur, vector<vector<int>> &result, vector<int> &elem) {
if (elem.size() == k) {
result.push_back(elem);
return;
}

for (int i = cur; i <= n; ++i) {
elem.push_back(i);
combine(n, k, i + 1, result, elem);
elem.pop_back();
}
}
};