#152 Combinations

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题目描述：

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

Have you met this question in a real interview?

Yes

Example

For example,
If n = 4 and k = 2, a solution is:
[[2,4],[3,4],[2,3],[1,2],[1,3],[1,4]]

题目思路：

这题我就用recursion做了，每次都挑一个数放入vector sofar中，看看是不是符合要求，如果符合要求就塞到ans中。

Mycode（AC = 16ms）：

class Solution {public:    /**     * @param n: Given the range of numbers     * @param k: Given the numbers of combinations     * @return: All the combinations of k numbers out of 1..n     */    vector<vector<int> > combine(int n, int k) {        // write your code here        vector<vector<int>> ans;        if (k == 0) return ans;                vector<int> sofar;        combine(ans, sofar, n, k, 1);        return ans;    }        void combine(vector<vector<int>>& ans,                 vector<int>& sofar,                 int n,                 int k,                 int start)    {        if (k == 0) {            ans.push_back(sofar);            return;        }                for (int i = start; i <= n; i++) {            sofar.push_back(i);            combine(ans, sofar, n, k - 1, i + 1);            sofar.pop_back();        }    }};

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