68--validate-binary-search-tree.java

class Solution {
    public boolean isValidBST(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode curr = root;
        TreeNode prev = null;
        while (curr != null || !stack.isEmpty()) {
            while (curr != null) {
                stack.push(curr);
                curr = curr.left;
            }
            curr = stack.pop();
            if (prev != null && curr.val <= prev.val) {
                return false;
            }
            prev = curr;
            curr = curr.right;
        }
        return true;
    }
} // TC: O(n), SC: O(n)