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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); } }
}

 

有效二叉查找树判断(java实现)