ValidateBinarySearchTree98.java

/**
 * 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.
 *
 * Example 1:
 *     2
 *    / \
 *   1   3
 * Binary tree [2,1,3], return true.
 *
 * Example 2:
 *     1
 *    / \
 *   2   3
 * Binary tree [1,2,3], return false.
 */

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class ValidateBinarySearchTree98 {
    public boolean isValidBST(TreeNode root) {
        return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    private boolean helper(TreeNode root, long min, long max) {
        if (root == null) return true;
        if (root.val <= min || root.val >= max) return false;
        return helper(root.left, min, root.val) && helper(root.right, root.val, max);
    }


    public boolean isValidBST2(TreeNode root) {
        if (root == null) return true;
        return isValidBST(root, new int[]{0, 0});
    }

    private boolean isValidBST(TreeNode root, int[] bounds) {
        bounds[0] = root.val;
        bounds[1] = root.val;
        if (root.left == null && root.right == null) {
            return true;
        }

        if (root.left != null) {
            int[] leftBounds = new int[]{0, 0};
            boolean left = isValidBST(root.left, leftBounds);
            if (!left || leftBounds[1] >= root.val) return false;
            bounds[0] = leftBounds[0];
        }

        if (root.right != null) {
            int[] rightBounds = new int[]{0, 0};
            boolean right = isValidBST(root.right, rightBounds);
            if (!right || rightBounds[0] <= root.val) return false;
            bounds[1] = rightBounds[1];
        }

        return true;
    }


    /**
     * https://discuss.leetcode.com/topic/46016/learn-one-iterative-inorder-traversal-apply-it-to-multiple-tree-questions-java-solution
     */
    public boolean isValidBST3(TreeNode root) {
        if (root == null) return true;
        Stack<TreeNode> stack = new Stack<>();
        TreeNode pre = null;
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if(pre != null && root.val <= pre.val) return false;
            pre = root;
            root = root.right;
        }
        return true;
    }

}