Combinations

public class Solution {    public ArrayList<ArrayList<Integer>> ret;    public ArrayList<ArrayList<Integer>> combine(int n, int k) {        // Start typing your Java solution below        // DO NOT write main() function        ret = new ArrayList<ArrayList<Integer>>();        find(0,k,1,n,new ArrayList<Integer>());        return ret;    }    private void find(int lev,int k,int now,int n,ArrayList<Integer> ans){        if(lev==k)            ret.add(new ArrayList<Integer>(ans));        for(int i=now;i<=n;i++){            ans.add(i);            find(lev+1,k,i+1,n,ans);            ans.remove(ans.size()-1);        }    }}

现在才切实感受到原来就是遍历树，剪掉不合适的。