LeetCode 652. Find Duplicate Subtrees

原题链接在这里：https://leetcode.com/problems/find-duplicate-subtrees/

题目：

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.

题解：

Do preorder serialization in subtree and see this serialization happens before. 

Use a HashMap to maintain the frequency of serialization.

Time Complexity: O(n).

Space: O(n).

AC Java:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        HashMap<String, Integer> hm = new HashMap<String, Integer>();
        List<TreeNode> res = new ArrayList<TreeNode>();
        preorder(root, hm, res);
        return res;
    }
    
    private String preorder(TreeNode root, HashMap<String, Integer> hm, List<TreeNode> res){
        if(root == null){
            return "#";
        }
        
        String key = root.val + "," + preorder(root.left, hm, res) + "," + preorder(root.right, hm, res);
        if(hm.containsKey(key) && hm.get(key) == 1){
            res.add(root);
        }
        
        hm.put(key, hm.getOrDefault(key, 0)+1);
        return key;
    }
}

类似Serialize and Deserialize BST, Serialize and Deserialize Binary Tree.