Word Search

题目大意：求一个字符串是否存在于由矩阵中，只能沿着上下左右四个方向查找

思路：深度遍历

class Solution {
public:
    bool exist(vector<vector<char> > &board, string word) {
        int m = board.size();
        int n = -1;
        if(m > 0) {
            n = board[0].size();
        }
        if(m == 0 || n == 0) {
            if(word.empty()) {
                return true;
            } else {
                return false;
            }
        }
        vector<vector<bool> > grid(m, vector<bool>(n, false));
        for(int i = 0; i < m; i++) {
            for(int j = 0; j < n; j++) {
                if(existAssist(board, grid, i, j, word, 0)) {
                    return true;
                }
            }
        }
        return false;
    }
private:
    bool existAssist(vector<vector<char> > &board, vector<vector<bool> > &grid, int i, int j, const string& word, int index) {
        if(index >= word.size()) {
            return true;
        }
        if(i >= 0 && i < board.size() && j >= 0 && j < board[0].size() && !grid[i][j] && board[i][j] == word[index]) {
            grid[i][j] = true;
            if(existAssist(board, grid, i + 1, j, word, index + 1)) {
                return true;
            }
            if(existAssist(board, grid, i - 1, j, word, index + 1)) {
                return true;
            }
            if(existAssist(board, grid, i, j + 1, word, index + 1)) {
                return true;
            }
            if(existAssist(board, grid, i, j - 1, word, index + 1)) {
                return true;
            }
            grid[i][j] = false;
        }
        return false;
    }
};