Leetcode 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.

confused what 

"{1,#,2,3}"

 means? >
read more on how binary tree is serialized on OJ.

Analysis:就是一个LinkedList, 然后进行中序遍历，得到的List再从前往后进行遍历，判断是否是递减的顺序

因为判断时候满足Binary Search Tree 的标准和中序遍历得到的结果相同

/**
* 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> res = new LinkedList<Integer>();
if(root == null)
return true;
helper(root,res);
Iterator iterator = res.iterator();
int position = 0;
int last = 0;
while(iterator.hasNext()){
int tem = (Integer)iterator.next();
position ++;
if(position == 1){
last = tem;
continue;
}
if(tem <= last)
return false;
}
return true;
}
public void helper(TreeNode tem,List<Integer> result ){
if(tem == null)
return;
helper(tem.left,result);
result.add(tem.val);
helper(tem.right,result);
}
}