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:

Input:
    2
   / \
  1   3
Output: true

Example 2:

    5
   / \
  1   4
     / \
    3   6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value
             is 5 but its right child's value is 4.


验证一棵树是否为二叉搜索树

java：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
        if (root == null)
            return true ;
        Stack<TreeNode> stack = new Stack<TreeNode>() ;
        TreeNode cur = root ;
        TreeNode pre = null ;
        boolean flag = true ;
        while(cur != null || !stack.isEmpty()){
            while(cur != null){
                stack.push(cur) ;
                cur = cur.left ;
            }
            TreeNode node = stack.pop() ;
            if (pre != null && node.val <= pre.val){
                flag = false ;
                break ;
            }
            pre = node ;
            cur = node.right ;
        }
        return flag ;
    }
}