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],
]

使用DFS，与Combination Sum类似的思路，只是判断size即可。

    public List> combine(int n, int k) {
    	ArrayList> at=new ArrayList>();
    	if(n<=0||n lt=new ArrayList();
    	dfs(n,k,1,lt,at);
    	return at;
    }
    private void dfs(int n,int k,int start,List lt,ArrayList> at){
    	if(lt.size()==k){
    		at.add(new ArrayList(lt));
    		return;
    	}
    	for(int i=start;i<=n;i++){
    		lt.add(i);
    		dfs(n,k,i+1,lt,at);
    		lt.remove(lt.size()-1);
    	}
    }