Your solution:

seems legit :) One thing I'd suggest here is to use ArrayDeque instead of Stack

Another possible solution:

Basically, for every subtree we have constrain, that every node in it should be in range (X, Y).

For root this range will be (-inf; +inf) - in other words, there could be any value in root.

For root's left subtree range will be (-inf, value-in-root), for right - (value-in-root, +inf).

Last thing - on each iteration we should check, that value in node is within this range, like so:

public boolean doInOrderTraversal(TreeNode root, int min, int max) {
    if(root == null) {
        return true;
    }
    if (root.val <= min || root.val >= max) {
        return false;
    }
 
    return doInOrderTraversal(root.left, min, root.val) && doInOrderTraversal(root.right, rot.val, max);
}

Your solution:

seems legit :) One thing I'd suggest here is to use ArrayDeque instead of Stack

Another possible solution:

Basically, for every subtree we have constrain, that every node in it should be in range (X, Y).

For root this range will be (-inf; +inf) - in other words, there could be any value in root.

For root's left subtree range will be (-inf, value-in-root), for right - (value-in-root, +inf).

Last thing - on each iteration we should check, that value in node is within this range, like so:

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

Your solution:

seems legit :) One thing I'd suggest here is to use ArrayDeque instead of Stack

Another possible solution:

Basically, for every subtree we have constrain, that every node in it should be in range (X, Y).

For root this range will be (-inf; +inf) - in other words, there could be any value in root.

For root's left subtree range will be (-inf, value-in-root), for right - (value-in-root, +inf).

Last thing - on each iteration we should check, that value in node is within this range, like so:

public boolean doInOrderTraversal(TreeNode root, int min, int max) {
    if(root == null) {
        return;
    }
    if (root.val <= min || root.val >= max) {
        return false;
    }
 
    return doInOrderTraversal(root.left, min, root.val) && doInOrderTraversal(root.right, rot.val, max);
}

Your solution:

seems legit :) One thing I'd suggest here is to use ArrayDeque instead of Stack

Another possible solution:

Basically, for every subtree we have constrain, that every node in it should be in range (X, Y).

For root this range will be (-inf; +inf) - in other words, there could be any value in root.

For root's left subtree range will be (-inf, value-in-root), for right - (value-in-root, +inf).

Last thing - on each iteration we should check, that value in node is within this range, like so:

public boolean doInOrderTraversal(TreeNode root, int min, int max) {
    if(root == null) {
        return true;
    }
    if (root.val <= min || root.val >= max) {
        return false;
    }
 
    return doInOrderTraversal(root.left, min, root.val) && doInOrderTraversal(root.right, rot.val, max);
}