95. Unique Binary Search Trees II

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<TreeNode> generateTrees(int n) {
         List<TreeNode> res=new ArrayList<TreeNode>();
        if(n==0){
            return res;
        }
        res=subTrees(1,n);
        return res;
    }
    
    public List<TreeNode> subTrees(int start,int end){
        List<TreeNode> listTree=new ArrayList<TreeNode>();　　　//每一次调用这个函数都会新创建一个list，保存子树，包括左子树和右子树
        List<TreeNode> leftTree;
        List<TreeNode> rightTree;
        if(start>end){
            listTree.add(null);//为子树添加节点，与循环体中的listTree不是一个
            return listTree;
        }     
        if(start==end){
            listTree.add(new TreeNode(start));//为子树添加节点
            return listTree;
        }
        for(int i=start;i<=end;i++){
            leftTree=subTrees(start,i-1);
            rightTree=subTrees(i+1,end);
            for(TreeNode lnode:leftTree){
                for(TreeNode rnode:rightTree){
                    TreeNode root=new TreeNode(i);
                    root.left=lnode;
                    root.right=rnode;
                    listTree.add(root);　//list存的是子树的根节点
                }
            }
        }
        return listTree;
    }
}
//整体结构　一个list保存树的根节点，根节点的左右指针（left和right）指向左右list,分别保存左右子树的根节点