力扣算法 Java 刷题笔记【二叉树篇】hot100（三）寻找重复的子树（中等） 框架思维训练 2

1. 计算一棵二叉树有多少个节点

地址: https://mp.weixin.qq.com/s/LJbpo49qppIeRs-FbgjsSQ
2021/12/01
做题反思：

int count(TreeNode root) {
    
    
	if (root == null) {
    
    
		return 0;
	}
	int left = count(root.left);
	int right = count(root.right);
	int res = left + right + 1;
	return res;
}

2. 寻找重复的子树（中等）

地址: https://leetcode-cn.com/problems/find-duplicate-subtrees/
2021/12/01
做题反思：通过拼接字符串的方式序列化二叉树

class Solution {
    
    
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
    
    
        traverse(root);
        return res;
    }
    // 集合泛型中用包装类
    HashMap<String, Integer> memo = new HashMap<>();
    // List 是接口， 应用其实现类
    LinkedList<TreeNode> res = new LinkedList<>();
    // 通过拼接字符串的方式序列化二叉树
    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 freq = memo.getOrDefault(subTree, 0);
        if (freq == 1) {
    
    
            res.add(root);
        } 
        memo.put(subTree, freq + 1);
        return subTree;
    }
}

/**
class Solution {
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        traverse(root);
        return res;
    }
    // 集合泛型中用包装类
    HashMap<String, Integer> memo = new HashMap<>();
    // List 是接口， 应用其实现类
    LinkedList<TreeNode> res = new LinkedList<>();
    // 通过拼接字符串的方式序列化二叉树
    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 freq = memo.getOrDefault(subTree, 0);
        if (freq == 1) {
            res.add(root);
        } 
        memo.put(subTree, freq + 1);
        return subTree;
    }
}
*/