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.

Example 2:

    1
   / \
  2   3

Binary tree[1,2,3], return false.

思路：中序遍历，吊炸天

public class Solution {
    public boolean isValidBST(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode curr = root;
        TreeNode prev = null;
        while (!stack.isEmpty()||curr!=null){
            if (curr!=null){
                stack.push(curr);
                curr = curr.left;
            }else{
                TreeNode p = stack.pop();
                if (prev != null && p.val <= prev.val) {                       
                       return false ;
                }
                prev = p;
                curr = p.right;
            }            
        }
        return true;
    }
}