Leetcode 98 Validate Binary Search Tree-----java

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 is binary search trees.

Example 1:

    2   /   1   3

Binary Tree [2,1,3] , return True.

Example 2:

    1   /   2   3

Binary tree [1,2,3] , return false.

Determine if a tree is a two-fork search tree.

The range can be determined. The main problem is that it appears at the maximum value of int, near the minimum value.

/*** Definition for a binary tree node. * public class TreeNode {* int val; * TreeNode left; * TreeNode rig Ht * TreeNode (int x) {val = x;} }*/ Public classSolution {intFlag1 = 0; intFlag2 = 0;  Public BooleanIsvalidbst (TreeNode root) {returnIsbst (Root,integer.max_value,integer.min_value); }     Public BooleanIsbst (TreeNode root,intMaxintmin) {        if(Root = =NULL )            return true; if(Root.val = = Integer.max_value && MAX = =root.val) {            if(Flag1 = = 0) {Flag1= 1; returnIsbst (Root.right,max,root.val) &&Isbst (root.left,root.val,min); }            Else                return false; }        if(Root.val = = Integer.min_value && MIN = =root.val) {            if(Flag2 = = 0) {Flag2= 1; returnIsbst (Root.right,max,root.val) &&Isbst (root.left,root.val,min); }            Else                return false; }        if(Root.val <=min | | root.val >=max)return false; returnIsbst (Root.right,max,root.val) &&Isbst (root.left,root.val,min); }}

So you can optimize it a little bit.

/*** Definition for a binary tree node. * public class TreeNode {* int val; * TreeNode left; * TreeNode rig Ht * TreeNode (int x) {val = x;} }*/ Public classSolution { Public BooleanIsvalidbst (TreeNode root) {returnDFS (Root, (Long) (Integer.min_value)-1, (Long) (Integer.max_value) +1); }    Private BooleanDFS (TreeNode root,LongGtLongLT) {        if(Root = =NULL)return true; if(root.val >= lt | | root.val <= GT)return false; returnDFS (root.left, GT, Root.val) &&dfs (root.right, root.val, lt); }}

Leetcode 98 Validate Binary Search Tree-----java