Leetcode | | 77, combinations

Problem:

Given-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],  [up],  [1,3],  [1,4],]

Hide TagsBacktrackingTest instructions: All permutation combinations of k numbers for output 1~n

Thinking:

(1) See the problem should think of using Dfs deep search method

(2) The difficulty of deep search is how to deal with the next step, here open a K-size array, record the deep search every step to get the number

(3) The time complexity is O (k*n), the space complexity is O (K)

Code

Class Solution {private:    vector<vector<int> > ret;    Vector<int> tmp;public:    vector<vector<int> > Combine (int n, int k) {        ret.clear ();        Tmp.resize (k);        DFS (1,n,k,1);       return ret;    } Protected:    void dfs (int dep, int n, int k,int start)    {       if (dep>k)       {           ret.push_back (TMP);           return;       }       for (int i=start;i<=n;i++)       {           tmp[dep-1]=i;           DFS (dep+1,n,k,i+1);}}    ;

Leetcode | | 77, combinations