Validate Binary 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 thanthe node's key.
- Both the left and right subtrees must also be binary search trees.
- A single node tree is a BST
Example
An example:
2
/ \
1 4
/ \
3 5
/**
* Definition of TreeNode:
* public class TreeNode {
* public int val;
* public TreeNode left, right;
* public TreeNode(int val) {
* this.val = val;
* this.left = this.right = null;
* }
* }
*/
public class Solution {
/**
* @param root: The root of binary tree.
* @return: True if the binary tree is BST, or false
*/
public boolean isValidBST(TreeNode root) {
// write your code here
if (root == null) {
return true;
}
if ((root.left == null) && (root.right == null)) {
return true;
} boolean left = false;
boolean right = false;
if (root.left == null) {
left = true;
}
else if ((root.left != null) && (root.left.val < root.val)) {
left = isValidBST(root.left);
}else {
return false;
} if (root.right == null) {
right = true;
}
else if ((root.right != null) && (root.right.val > root.val)) {
right = isValidBST(root.right);
}else {
return false;
} return left && right;
}
}