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【LeetCode】Combinations

Combinations

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],]

 

递推点:加入i后,下一个加入的元素需要遍历i+1~n

因此可以基于k做递归。

base case: k=1,把所有的元素都逐个加入集合。

 

class Solution {public:    vector<vector<int> > combine(int n, int k) {        vector<vector<int> > result;        vector<int> cur;        Helper(result, cur, 1, n, k);        return result;    }    void Helper(vector<vector<int> >& result, vector<int> cur, int begin, int n, int k)    {        if(k == 1)        {            for(int i = begin; i <= n; i ++)            {                cur.push_back(i);                result.push_back(cur);                cur.pop_back();            }        }        else        {            for(int i = begin; i <= n; i ++)            {                cur.push_back(i);                Helper(result, cur, i+1, n, k-1);                cur.pop_back();            }        }    }};

【LeetCode】Combinations