# 77. Combinations
dfs深搜，99%，需要处理一下的是剪枝，可以在一定程度上提高效率。
class Solution {
public:
vector<vector<int>> combine(int n, int k) {
vector<vector<int>> ans;
vector<int> tpath;
dfs(1, n, k, ans, tpath);
return ans;
}
void dfs(int cur, int &n, int k, vector<vector<int>> &ans, vector<int> tpath) {
if (k == 0) {
ans.push_back(tpath);
return ;
} else {
for (int i = cur; i <= n - k + 1; i++) {
tpath.push_back(i);
dfs(i + 1, n, k - 1, ans, tpath);
tpath.pop_back();
}
}
}
};
