LeetCode() Word Search II

class Solution {
    class TrieNode {
public:
    // Initialize your data structure here.
    TrieNode() {
        for(int i=0;i<26;i++)
            next[i]=NULL;
        isString = false;
    }
    TrieNode *next[26];
    bool isString;
};

class Trie {
public:
    Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    void insert(string word) {
        TrieNode* p=root;
        for(int i=0;i<word.size();i++){
            if(p->next[word[i]-‘a‘] == NULL)
                p->next[word[i]-‘a‘]=new TrieNode();
            p=p->next[word[i]-‘a‘];
        }
        p->isString=true;
        
    }

    // Returns if the word is in the trie.
    bool search(string word) {
        TrieNode* p=root;
        for(int i=0;i<word.size();i++){
            if(p->next[word[i]-‘a‘] == NULL)    return false;
            p=p->next[word[i]-‘a‘];
        }
        if(p->isString == true)
            return true;
        return false;
     //  return p->isString;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    bool startsWith(string prefix) {
        TrieNode* p=root;
        for(int i=0;i<prefix.size();i++){
            if(p->next[prefix[i]-‘a‘] == NULL)    return false;
            p=p->next[prefix[i]-‘a‘];
        }
        return true;
    }

private:
    TrieNode* root;
};

// Your Trie object will be instantiated and called as such:
// Trie trie;
// trie.insert("somestring");
// trie.search("key");
public:
    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
        vector<string> res;
        Trie s;
        for(auto i:words)
        {
            if(s.search(i))
            {
                s.insert(i);
                res.push_back(i);
            }
            else if(exist(board,i))
            {
                res.push_back(i);
                s.insert(i);
            }
        }
        sort(res.begin(),res.end());
        return res;
    }
    bool exist(vector<vector<char>>& board,string word){
        for(int i=0;i<board.size();i++)
            for(int j=0;j<board[0].size();j++)
                if(exist(board,word,i,j,0))     return true;
        return false;
    }
    bool exist(vector<vector<char>>& board,string word,int x,int y,int pos)
    {
        if(pos == word.size())  return true;
        if(x<0 || x>=board.size() || y<0 || y>=board[0].size()) return false;
        if(word[pos] == board[x][y])
        {
            char c=board[x][y];
            board[x][y]=‘#‘;
            bool res=exist(board,word,x+1,y,pos+1)||exist(board,word,x-1,y,pos+1)||exist(board,word,x,y+1,pos+1)||exist(board,word,x,y-1,pos+1);
            board[x][y]=c;
            return res;
        }
        return false;
    }
};