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[!info] related notes 算法面试题型 MOC 二叉树高频题

Binary Tree Serialization (Level-Order)

LeetCode: https://leetcode.cn/problems/serialize-and-deserialize-binary-tree/

思路

• BFS 输出节点值。
• 遇到空孩子也输出占位符（例如 #），否则结构会丢。
• 反序列化时用队列逐层挂孩子。

Java Template

import java.util.*;

public class Codec {
    private static final String SEP = ",";
    private static final String NIL = "#";

    public String serialize(TreeNode root) {
        if (root == null) return NIL;
        StringBuilder sb = new StringBuilder();
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while (!q.isEmpty()) {
            TreeNode cur = q.poll();
            if (cur == null) {
                sb.append(NIL).append(SEP);
                continue;
            }
            sb.append(cur.val).append(SEP);
            q.add(cur.left);
            q.add(cur.right);
        }
        return sb.toString();
    }

    public TreeNode deserialize(String data) {
        if (data == null || data.isEmpty() || data.equals(NIL)) return null;
        String[] parts = data.split(SEP);
        int idx = 0;

        TreeNode root = node(parts[idx++]);
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);

        while (!q.isEmpty()) {
            TreeNode cur = q.poll();
            if (cur == null) continue;
            if (idx < parts.length) {
                cur.left = node(parts[idx++]);
                q.add(cur.left);
            }
            if (idx < parts.length) {
                cur.right = node(parts[idx++]);
                q.add(cur.right);
            }
        }
        return root;
    }

    private TreeNode node(String s) {
        if (s.equals(NIL) || s.isEmpty()) return null;
        return new TreeNode(Integer.parseInt(s));
    }
}

Related

• 二叉树先序序列化
• 二叉树高频题