[LeetCode] Word Search

Given a 2D board and a word, find if the word exists in the grid.

The word can be constructed from letters of sequentially adjacent cell, where "adjacent" cells are those horizontally or vertically neighboring. The same letter cell may not be used more than once.

For example,
Given board =

[
  ["ABCE"],
  ["SFCS"],
  ["ADEE"]
]

word = "ABCCED", -> returns true,
word = "SEE", -> returns true,
word = "ABCB", -> returns false. Note: 1) Translating string to char* will facilitate solving the problem. 2) Set element of the original matrix to NULL to mark its visited.

class Solution {
public:
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.c_str(), i, j))
                return true;
    return false;
}

bool exist(vector<vector<char> > &board, const char* word, int row, int col) {  
    if (*word != board[row][col] || board[row][col] == NULL)
        return false;

    if (*(word + 1) == NULL)
        return true;

    char t = board[row][col];
    board[row][col] = NULL;

    if (row > 0 && exist(board, word + 1, row - 1, col) 
        || col > 0 && exist(board, word + 1, row, col - 1)
        || row < board.size() - 1 && exist(board, word + 1, row + 1, col)
        || col < board[0].size() - 1 && exist(board, word + 1, row, col + 1))
        return true;

    board[row][col] = t;

    return false;
  }
};