79. Word Search LeetCode

题意：给一个存有字母的二维矩阵和一个单词，问单词是否可以是二维矩阵的一条路径上的字母组成的，路径移动方向是4个方向，上下左右。
题解：dfs即可。

class Solution {
public:
    int dir[4][2] = {{1,0},{-1,0},{0,1},{0,-1}};
    bool dfs(vector<vector<char>>& board, string word,int i,int j)
    {
        if(word.length() < 1) return true;
        char c = board[i][j];
        int n = board.size();
        int m = board[0].size();
        board[i][j] = '\0';
        bool flag = false;
        for(int k = 0; k < 4; k++)
        {
            int tx = i + dir[k][0];
            int ty = j + dir[k][1];
            if(tx < 0 || ty < 0 || tx >= n || ty >= m || board[tx][ty] != word[0]) continue;
            flag |= dfs(board,word.substr(1),tx,ty);
        }
        if(flag) return true;
        board[i][j] = c;
        return false;
    }
    bool exist(vector<vector<char>>& board, string word) {
        int n = board.size();
        int m = board[0].size();
        for(int i = 0; i < n; i++)
            for(int j = 0; j < m; j++)
                if(board[i][j] == word[0] && dfs(board,word.substr(1),i,j))
                    return true;
        return false;
    }
};