Duplicate subtree in Binary Tree

Given a binary tree, find out whether it contains a duplicate sub-tree of size two or more, or not.

• Time : O(n * n)
• Space : O(n)

class Solution {
    Map<Integer, List<Node>> mp = new HashMap<>();
    void travel(Node node) {
        if(node == null) return;
        if(node.left != null || node.right != null) {
            List<Node> list = mp.getOrDefault(node.data, new ArrayList<>());
            list.add(node);
            mp.put(node.data, list);
            travel(node.left);
            travel(node.right);
        }
    }
    boolean equal(Node a, Node b) {
        if(a == null || b == null) return a == null && b == null;
        return a.data == b.data && equal(a.left, b.left) && equal(a.right, b.right);
    }
    int dupSub(Node root) {
        mp.clear();
        travel(root);
        for(List<Node> nodes : mp.values()) {
            int n = nodes.size();
            for(int i = 0; i < n; i++) {
                for(int j = i + 1; j < n; j++) {
                    if(equal(nodes.get(i), nodes.get(j)))
                        return 1;
                }
            }
        }

        return 0;
    }
}