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);
        }
    }
}