652.FindDuplicateSubtrees.java

import java.util.HashMap;
import java.util.LinkedList;

/**
 * Definition for a binary tree node. public class TreeNode { int val; TreeNode
 * left; TreeNode right; TreeNode() {} TreeNode(int val) { this.val = val; }
 * TreeNode(int val, TreeNode left, TreeNode right) { this.val = val; this.left
 * = left; this.right = right; } }
 */
class Solution {

    HashMap<String, Integer> memo = new HashMap<>();
    LinkedList<TreeNode> res = new LinkedList<>();

    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {

        traverse(root);
        return res;
    }

    public String traverse(TreeNode root) {
        if (root == null)
            return "#";

        String left = traverse(root.left);
        String right = traverse(root.right);

        String subTree = left + "," + right + "," + root.val;

        int freg = memo.getOrDefault(subTree, 0);

        if (freg == 1) {
            res.add(root);
        }

        memo.put(subTree, freg + 1);
        return subTree;
    }
}