Find Duplicate Subtrees

Given a binary tree, return all duplicate subtrees. For each kind of duplicate subtrees, you only need to return the root node of any one of them.

Two trees are duplicate if they have the same structure with same node values.

Example 1:

        1
       / \
      2   3
     /   / \
    4   2   4
       /
      4

The following are two duplicate subtrees:

      2
     /
    4

and

    4

Therefore, you need to return above trees' root in the form of a list.

Here is a solution.

class Solution {
    private final List<TreeNode> duplicates = new ArrayList<>();

    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        // a node in a binary tree is: node, node.left and node.right
        // we need to iterate through the nodes and build key for each node, then as we are building the keys and a hash map, we check for duplicate keys, if duplicate key is found, we add that node into list
        // 1 -> 2 ->null,null
        // 1 -> 2 ->null,null
        find(root);
        return duplicates;
    }

    private Map<String, Integer> keys = new HashMap<>();

    public String find(TreeNode node) {
        if (node == null) return "-";
        String key = String.valueOf(node.val) + find(node.left) + find(node.right);
        System.out.println(key);
        if (keys.containsKey(key) && keys.get(key) == 0) {
            System.out.println("adding...");
            duplicates.add(node);
            keys.put(key, 1);
        } else {
            if (!keys.containsKey(key)) {
                keys.put(key, 0);
            }
        }
        return key;
    }
}