leetCode :Validate Binary Search Tree
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给出一个二叉树,询问该二叉树是不是BST(二分查找树)
做法:
假设该树是一颗二分查找树,那么对其进行先序遍历,遍历的顺序是有序。
在遍历的过程中只需记录前一个节点的值和当前节点进行比较,
如果 前一个节点的值 >= 当前节点的值 ,那么就不是BST了
/** * Definition for binary tree * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */public class Solution { static int preRetVal = Integer.MIN_VALUE; static boolean flag = true; public boolean isValidBST(TreeNode root) { preRetVal = Integer.MIN_VALUE; flag = true; preOrder(root); return flag; } public void preOrder(TreeNode node){ if(node != null && flag){ preOrder(node.left); if(flag == false){ return; } if(preRetVal >= node.val){ flag = false; return ; } preRetVal = node.val; preOrder(node.right); } } }
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