【LeetCode】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 than the node's key.

Both the left and right subtrees must also be binary search trees.

package com.leetcode.ValidateBinarySearchTree;

public class Solution {
public boolean isValidBST(TreeNode root) {
if(root==null)
return true;
boolean left=false;
boolean right=false;
if(root.left!=null){
if(isLeftValid(root.left,root.val))
left = isValidBST(root.left);
}else{
left=true;
}
if(root.right!=null){
if(isRightValid(root.right,root.val))
right = isValidBST(root.right);
}else{
right=true;
}
return left&&right;
}

private boolean isRightValid(TreeNode right, int val) {

if(right!=null){
if(right.val<=val)
return false;
return isRightValid(right.left,val)&&isRightValid(right.right, val);
}else{
return true;
}

}

private boolean isLeftValid(TreeNode left, int val) {
if(left!=null){
if(left.val>=val)
return false;
return isLeftValid(left.left,val)&&isLeftValid(left.right, val);
}else{
return true;
}

}

}