LeetCode 677 Map Sum Pairs (Trie)

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本文详细解析了LeetCode上MapSum Pairs问题的解决方案，通过使用Trie树和HashMap来实现高效的插入和求和操作。文章展示了如何利用Trie树存储字符串键，并在每个节点维护一个计数器，以便快速计算以特定前缀开始的所有键值对的总和。

Implement a MapSum class with insert, and sum methods.

For the method insert, you'll be given a pair of (string, integer). The string represents the key and the integer represents the value. If the key already existed, then the original key-value pair will be overridden to the new one.

For the method sum, you'll be given a string representing the prefix, and you need to return the sum of all the pairs' value whose key starts with the prefix.

Example 1:

Input: insert("apple", 3), Output: Null
Input: sum("ap"), Output: 3
Input: insert("app", 2), Output: Null
Input: sum("ap"), Output: 5

题目链接：https://leetcode.com/problems/map-sum-pairs/

题目分析：裸Trie，用map记录(string, integer) pair

49ms，时间击败96%

class MapSum {

    /** Initialize your data structure here. */
    public class Trie {
        public Trie[] nxt;
        public int cnt;
        Trie() {
            this.nxt = new Trie[26];
            this.cnt = 0;
        }
    }
    
    public Trie root;
    public Map<String, Integer> mp;
    
    public MapSum() {
        this.root = new Trie();
        this.mp = new HashMap<>();
    }
    
    public void insert(String key, int val) {
        int pre = 0;
        if (this.mp.containsKey(key)) {
            pre = this.mp.get(key);
        }
        this.mp.put(key, val);
        Trie p = this.root;
        for (int i = 0; i < key.length(); i++) {
            int idx = key.charAt(i) - 'a';
            if (p.nxt[idx] == null) {
                p.nxt[idx] = new Trie();
            }
            p = p.nxt[idx];
            p.cnt += val - pre;
        }
    }
    
    public int sum(String prefix) {
        Trie p = this.root;
        for (int i = 0; i < prefix.length(); i++) {
            int idx = prefix.charAt(i) - 'a';
            if (p.nxt[idx] == null) {
                return 0;
            }
            p = p.nxt[idx];
        }
        return p.cnt;
    }
}

/**
 * Your MapSum object will be instantiated and called as such:
 * MapSum obj = new MapSum();
 * obj.insert(key,val);
 * int param_2 = obj.sum(prefix);
 */