677. Map Sum Pairs(通过Trie求前缀和)

题意：通过Trie求前缀和，相同字符串取最新的值。

class TrieNode{
public:
    int val;
    TrieNode *nex[26];
    TrieNode(){
        val = 0;
        for(int i = 0; i < 26; ++i){
            nex[i] = NULL;
        }
    }
};
class MapSum {
public:
    TrieNode *root;
    map<string, int> mp;
    /** Initialize your data structure here. */
    MapSum() {
        root = new TrieNode();
    }
    void insert(string key, int val) {
        TrieNode *p = root;
        int len = key.size();
        int add = val - mp[key];
        for(int i = 0; i < len; ++i){
            if(p -> nex[key[i] - 'a'] == NULL){
                p -> nex[key[i] - 'a'] = new TrieNode();
            }
            p = p -> nex[key[i] - 'a'];
            p -> val += add;
        }
        mp[key] = val;
    }
    
    int sum(string prefix) {
        TrieNode *p = root;
        int len = prefix.size();
        for(int i = 0; i < len; ++i){
            if(p -> nex[prefix[i] - 'a'] == NULL) return 0;
            p = p -> nex[prefix[i] - 'a'];
        }
        return p -> val;
    }
};

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