mission-peace/interview · error · IllegalArgumentException

Duplicate date

Error message

Duplicate date 

What it means

RedBlackTree.insert does not allow duplicate keys; when the descent reaches a node whose data equals the value being inserted it throws IllegalArgumentException("Duplicate date " + data). This is a deliberate invariant of this tree implementation rather than an accidental bug (note the 'date' typo in the message).

Solutions

1. Check search(data) != null before calling insert and skip/update instead of re-inserting
2. Deduplicate the input collection before bulk loading into the tree
3. Catch IllegalArgumentException around insert if duplicates should be silently ignored

Example fix

// before
tree.insert(date);
// after
if (tree.search(date) == null) {
    tree.insert(date);
}

if (tree.search(data) != null) { /* already present, skip */ } else { tree.insert(data); }

try {
    tree.insert(data);
} catch (IllegalArgumentException e) {
    // duplicate key, ignore or update
}

When it happens

Trigger: Calling insert(data) with a value already present in the tree, including the initial insert(root, data) call and recursive insert(root, node, data) descents.

Common situations: Loading a dataset with repeated keys (e.g. duplicate dates in a time-series index), re-inserting an already processed record, or retry logic that replays an insert without checking membership.

Related errors

• Not same length input
• Cloning non directed graph
• No vertex in the graph
• Key already exists

AI-assisted analysis of mission-peace/interview@94be5deb0c (2026-09-08). Data as JSON: /api/errors/c36d187b08019a2d. Report an issue: GitHub.

Appendix: source

Thrown at src/com/interview/tree/RedBlackTree.java:190

        } else {
            return false;
        }
    }

    private Node insert(Node parent, Node root, int data) {
        if(root  == null || root.isNullLeaf) {
            //if parent is not null means tree is not empty
            //so create a red leaf node
            if(parent != null) {
                return createRedNode(parent, data);
            } else { //otherwise create a black root node if tree is empty
                return createBlackNode(data);
            }
        }

        //duplicate insertion is not allowed for this tree.
        if(root.data == data) {
            throw new IllegalArgumentException("Duplicate date " + data);
        }
        //if we go on left side then isLeft will be true
        //if we go on right side then isLeft will be false.
        boolean isLeft;
        if(root.data > data) {
            Node left = insert(root, root.left, data);
            //if left becomes root parent means rotation
            //happened at lower level. So just return left
            //so that nodes at upper level can set their
            //child correctly
            if(left == root.parent) {
                return left;
            }
            //set the left child returned to be left of root node
            root.left = left;
            //set isLeft to be true
            isLeft = true;
        } else {

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