[刷题]Validate Binary Search Tree

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[LintCode]Validate Binary Search Tree

/** * Definition of TreeNode: * public class TreeNode { *     public int val; *     public TreeNode left, right; *     public TreeNode(int val) { *         this.val = val; *         this.left = this.right = null; *     } * } */public class Solution {    /**     * @param root: The root of binary tree.     * @return: True if the binary tree is BST, or false     */    boolean isFirstNode = true;    int lastVal;    public boolean isValidBST(TreeNode root) {        // 2015-4-1 inoder traversal        if (root == null) {            return true;        }                if (!isValidBST(root.left)) {            return false;        }                if (!isFirstNode && lastVal >= root.val) {            return false;        }        isFirstNode = false;        lastVal = root.val;        if (!isValidBST(root.right)) {            return false;        }                 return true;    }}

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