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。