0652. Find Duplicate Subtrees

https://leetcode.com/problems/find-duplicate-subtrees

Description

Given the root of 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 the same node values.

Example 1:

**Input:** root = [1,2,3,4,null,2,4,null,null,4]
**Output:** [[2,4],[4]]

Example 2:

**Input:** root = [2,1,1]
**Output:** [[1]]

Example 3:

**Input:** root = [2,2,2,3,null,3,null]
**Output:** [[2,3],[3]]

Constraints:

• The number of the nodes in the tree will be in the range [1, 10^4]

• -200 <= Node.val <= 200

ac1: serialize subtree

serialize subtree as a hash of the tree

https://leetcode.com/problems/find-duplicate-subtrees/discuss/106011/Java-Concise-Postorder-Traversal-Solution

class Solution {
    List<TreeNode> duplicate;
    Map<String, Integer> map;

    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        duplicate = new ArrayList<>();
        map = new HashMap<>();

        process(root);

        return duplicate;
    }

    private String process(TreeNode node) {
        if (node == null) return "#";

        // String concatation is O(n), so overall is O(n^2)
        String hash = node.val + "," + process(node.left) + "," + process(node.right);

        map.put(hash, map.getOrDefault(hash, 0) + 1);

        if (map.get(hash) == 2) {
            // Only add one node for multiple duplicate subtrees.
            duplicate.add(node);
        }

        return hash;
    }
}