_652.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.TreeNode;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Queue;
import java.util.Set;

/**
 * 652. 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.
 */
public class _652 {
    public static class Solution1 {

        /**credit: https://discuss.leetcode.com/topic/97584/java-concise-postorder-traversal-solution*/

        /**
         * You don't actually need to check if every other tree is a duplicate of current node,
         * just when you go through each node, you'll see whether there's already one in the map,
         * since map.containsKey() checks this TreeNode.
         */
        public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
            List<TreeNode> res = new LinkedList<>();
            postorder(root, new HashMap<>(), res);
            return res;
        }

        private String postorder(TreeNode curr, HashMap<String, Integer> map, List<TreeNode> res) {
            if (curr == null) {
                return "#";
            }
            String serial = curr.val + "," + postorder(curr.left, map, res) + "," + postorder(curr.right, map, res);
            if (map.getOrDefault(serial, 0) == 1) {
                res.add(curr);
            }
            map.put(serial, map.getOrDefault(serial, 0) + 1);
            return serial;
        }
    }
}