leetcode-652寻找重复的子树

Posted 2021-12-09 JasonLee-后厂村程序员

【leetcode-652】寻找重复的子树

题目描述

给定一棵二叉树，返回所有重复的子树。对于同一类的重复子树，你只需要返回其中任意一棵的根结点即可。

两棵树重复是指它们具有相同的结构以及相同的结点值。

题解

/**
 * 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 {

    List<TreeNode> res = new ArrayList<>();
    Map<String, Integer> map = new HashMap<>();

    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        helper(root);
        return res;
    }

    public String helper(TreeNode root) {
        if (root == null) return "";
        String key = root.val + "," + helper(root.left) + "," + helper(root.right);
        map.put(key, map.getOrDefault(key, 0) + 1);
        int value = map.get(key);
        if (value == 2) {
            res.add(root);
        }
        return key;
    }
}

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