LintCode 132. Word Search II -- Trie

Description
Given a matrix of lower alphabets and a dictionary. Find all words in the dictionary that can be found in the matrix. A word can start from any position in the matrix and go left/right/up/down to the adjacent position.

Example
Given matrix:
doaf
agai
dcan
and dictionary:

{“dog”, “dad”, “dgdg”, “can”, “again”}

return {“dog”, “dad”, “can”, “again”}

思路：利用给定的字典构建trie树，减少重复遍历，同时对给定的矩阵进行深度优先搜索，判断给定的字典中的字符串是否存在于矩阵中。
C++ 代码如下：

class TrieNode{
public:
    TrieNode* child[26];
    string str;
    TrieNode(){
        str = "";
        memset(child, 0, sizeof(child));
    }
};
class Solution {
public:
    /**
     * @param board: A list of lists of character
     * @param words: A list of string
     * @return: A list of string
     */
    vector<string> results;
    TrieNode* root;
    void insert(TrieNode* p, string s){
        for(int i=0; i<s.size(); ++i){
            if(p->child[s[i]-'a']==0) p->child[s[i]-'a'] = new TrieNode();
            p = p->child[s[i]-'a'];
        }
        p->str = s;
    }
    void search(vector<vector<char>> &board, vector<vector<bool>> &mask, TrieNode* p, int x, int y){
        if(p->str != ""){
            results.push_back(p->str);
            p->str = "";
        }
        mask[x][y] = false;
        int row = board.size(), col = board[0].size();
        if(x+1<row && mask[x+1][y] && p->child[board[x+1][y]-'a']) search(board, mask, p->child[board[x+1][y]-'a'], x+1, y);
        if(y+1<col && mask[x][y+1] && p->child[board[x][y+1]-'a']) search(board, mask, p->child[board[x][y+1]-'a'], x, y+1);
        if(x-1>=0 && mask[x-1][y] && p->child[board[x-1][y]-'a']) search(board, mask, p->child[board[x-1][y]-'a'], x-1, y);
        if(y-1>=0 && mask[x][y-1] && p->child[board[x][y-1]-'a']) search(board, mask, p->child[board[x][y-1]-'a'], x, y-1);
        mask[x][y] = true;
    }
    vector<string> wordSearchII(vector<vector<char>> &board, vector<string> &words) {
        // write your code here
        if(board.size()==0) return results;
        root = new TrieNode();
        for(int i=0; i<words.size(); ++i) insert(root, words[i]);
        vector<vector<bool> > mask(board.size(), vector<bool>(board[0].size(), true));
        for(int i=0; i<board.size(); ++i)
            for(int j=0; j<board[0].size(); ++j)
                if(root->child[board[i][j]-'a'])    search(board, mask, root->child[board[i][j]-'a'], i, j);
        return results;
    }
};