题目描述

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.

题目链接

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

方法思路

class Solution {
    //Runtime: 13 ms, faster than 96.49%
    //Memory Usage: 61.5 MB, less than 30.00%
    Map<String, Integer> count;
    List<TreeNode> ans;
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        count = new HashMap();
        ans = new ArrayList();
        collect(root);
        return ans;
    }
 //为什么要转换成字符串的形式再进行添加判定呢？，不是很懂。tricky!
    public String collect(TreeNode node) {
        if (node == null) return "#";
        String serial = node.val + "," + collect(node.left) + "," + collect(node.right);
        count.put(serial, count.getOrDefault(serial, 0) + 1);
        if (count.get(serial) == 2)
            ans.add(node);
        return serial;
    }
}