LeetCode | 77. Combinations

题目：

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example:

Input: n = 4, k = 2
Output:
[[2,4],[3,4],[2,3],[1,2],[1,3],[1,4],
]

代码：

class Solution {
public:vector<vector<int>> combine(int n, int k) {vector<vector<int>> res;if (n < k)return res;int ori_k = k;if (k > n / 2)k = n - k;set<set<int>> cur_res;set<int> tmp;cur_res.insert(tmp);for (int i = 0; i < k; i++){set<set<int>> update_res;for (set<set<int>>::iterator iter = cur_res.begin(); iter != cur_res.end(); iter++){for (int j = 1; j <= n; j++){set<int> cbnt = *iter;cbnt.insert(j);if (cbnt.size() == i + 1){update_res.insert(cbnt);}}}cur_res = update_res;}if (ori_k != k){set<int> total;for (int i = 1; i <= n; i++)total.insert(i);set<set<int>> update_res;for (set<set<int>>::iterator iter = cur_res.begin(); iter != cur_res.end(); iter++){set<int> cur = *iter;set<int> cplm = total;for (set<int>::iterator jter = cur.begin(); jter != cur.end(); jter++) {cplm.erase(*jter);}update_res.insert(cplm);}cur_res = update_res;}for (set<set<int>>::iterator iter = cur_res.begin(); iter != cur_res.end(); iter++){set<int> cur_set = *iter;vector<int> cur_nums;for (set<int>::iterator jter = cur_set.begin(); jter != cur_set.end(); jter++){cur_nums.push_back(*jter);}res.push_back(cur_nums);}return res;}
};

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