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.

 

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
    	ArrayList<Integer> arrayList = new ArrayList<Integer>();
    	arrayList.add(null);
        return solve(root, arrayList);
    }

	private boolean solve(TreeNode root, ArrayList<Integer> arrayList) {
		if (root == null) {
			return true;
		}
		boolean left = solve(root.left, arrayList);
		if (arrayList.get(arrayList.size()-1) != null && root.val<=arrayList.get(arrayList.size()-1)) {
			return false;
		}
		arrayList.add(root.val);
		boolean right = solve(root.right, arrayList);
		return left && right;
	}

}