有效二叉查找树判断（java实现）

leetcode 原题 ：（即判断二叉树是否为二叉查找树）

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

下面采用java实现：

public class TreeNode {

        int val;

        TreeNode left;

        TreeNode right;

      TreeNode(int x) { val = x; }

  }

 

public class ValidateBST{

    boolean flag=true;

    TreeNode pre=null;//保存前驱节点

    public boolean isValidBST(TreeNode root) {

         dfs(root);

         return flag;

    }
　　//采用中序遍历，如果前节点不小于当前节点的值，则不符合BST的条件

    private void dfs(TreeNode root){

        if(root!=null){

            dfs(root.left);

            if(pre!=null&&root.val<=pre.val)flag= false;

            pre=root;

            dfs(root.right);

        }

    }
}