【Leetcode】Validate Bunary Search Tree (Tree Judge)

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.

helper用于判断每一个节点的左界和右界，

往左走的时候，左界不变，而右界为当前节点，

往右走的时候，左界为当前节点，而右界不变。

压栈思路：先将全部左节点压入，然后弹出左节点压右节点。在压栈的过程中先比较根节点是否为空，再判断根节点的左右界。

    public boolean isValidBST(TreeNode root) {return helper(root, Integer.MIN_VALUE, Integer.MAX_VALUE);    }    private boolean helper(TreeNode root, int min, int max) {if (root == null)return true;if(root.val<=min || root.val>=max)    return false;boolean left = helper(root.left, min, root.val);boolean right = helper(root.right, root.val, max);return left&&right;    }