老师，我自己试了一下是成功的，但是在leetcode是失败，我想问一下是我的代码有哪里错了吗

import java.util.TreeMap;

public class Test {
    public static void main(String[] args) {
        MapSum sum = new MapSum();
        sum.insert("apple", 5);
        sum.insert("app", 3);
        System.out.println(sum.sum("a"));
    }
}


class MapSum {

    private class Node {
        public boolean isWord;
        public int value;
        public TreeMap<Character, Node> next;

        public Node(boolean isWord, int value) {
            this.isWord = isWord;
            this.value = value;
            next = new TreeMap<>();
        }

        public Node(boolean isWord) {
            this(isWord, 0);
        }

        public Node() {
            this(false, 0);
        }

        public Node set(int value) {
            this.value = value;
            return this;
        }
    }

    private Node root;

    /** Initialize your data structure here. */
    public MapSum() {
        root = new Node();
    }

    public void insert(String key, int val) {
        Node cur = root;
        for(int i = 0; i < key.length(); i++) {
            char c = key.charAt(i);
            if(cur.next.get(c) == null)
                cur.next.put(c, new Node(false, val));
            else
                cur.next.put(c, cur.next.get(c).set(cur.next.get(c).value + val));
            cur = cur.next.get(c);
        }
        if(!cur.isWord) {
            cur.isWord = true;
        }
    }

    public int sum(String prefix) {
        Node cur = root;
        for(int i = 0; i < prefix.length(); i++) {
            char c = prefix.charAt(i);
            if(cur.next.get(c) == null)
                return 0;
            cur = cur.next.get(c);
        }
        return cur.value;
    }

}