leetCode练习（77）

题目：Combinations          

难度：medium

问题描述：

Given two integers n and k, return all possible combinations ofk 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],
]

解题思路：使用DFS，回溯求解。注意添加判断：剩余字符数小于剩余步数时，不在搜索。可以极大减少迭代次数。

具体代码如下：

public List> combine(int n, int k) {
	   List> reslist=new ArrayList>();
       diedai(new ArrayList(),0,k,1,n,reslist);
       return reslist;
    }
	//可用字符从start到end
	//已经进行step步，一共需要k步
	public void diedai(ArrayList list,int step,int k,int start,int end,List> reslist){
		if((k-step)>(end-start+1)){
			return;
		}
		if(step==k){
			List temp = new ArrayList<>(list);
        	reslist.add(temp);
        	return;
		}
		for(int i=start;i<=end;i++){	
			list.add(i);
			diedai(list,step+1,k,i+1,end,reslist);
			list.remove(list.size()-1);
		}
	}