98. Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:
The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.

Example 1:

2
/ \
1   3

Binary tree 

[2,1,3]

,
return true.

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean isValidBST(TreeNode root) {
List<Integer> re=new ArrayList<Integer>();
fun(re, root);
for(int i=0;i<re.size()-1;++i){
if(re.get(i)>=re.get(i+1))
return false;
}
return true;
}
static void fun(List<Integer> re,TreeNode root){
if(root!=null){
fun(re, root.left);
re.add(root.val);
fun(re, root.right);
}
}
}