Effective binary lookup tree judgment (Java implementation)

Leetcode Original question: (That is, to determine whether the binary tree is a two-fork search tree)

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

The following Java implementations are used:

 Public classTreeNode {intVal;        TreeNode left;      TreeNode right; TreeNode (intX) {val =x;} }  Public classvalidatebst{Booleanflag=true; TreeNode Pre=NULL;//Save precursor node Public BooleanIsvalidbst (TreeNode root) {DFS (root); returnFlag; }
In the middle order traversal, if the former node is not less than the value of the current node, it does not conform to the conditions of BSTPrivate voiddfs (TreeNode root) {if(root!=NULL) {DFS (root.left); if(pre!=NULL&&root.val<=pre.val) flag=false; Pre=Root;        DFS (Root.right); }    }
}

Effective binary lookup tree judgment (Java implementation)