Validate_Binary_Search_Tree.java

//Leetcode 98. Validate Binary Search Tree
//Question - https://leetcode.com/problems/validate-binary-search-tree/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode(int x) { val = x; }
 * }
 */
classSolution {
publicbooleanisValidBST(TreeNoderoot) {
Stack<TreeNode> stk = newStack<TreeNode>();

TreeNodeprev = null;

while(root!=null || stk.size()!=0){

//keep going left
while(root!=null){
stk.push(root);
root = root.left;
 }

root = stk.pop();
//if(prev!=null) System.out.println(prev.val+" "+root.val);
if(prev!=null && prev.val>=root.val) returnfalse;
prev = root;
root = root.right;

 }

returntrue;

 }
}