LeetCode "Word Search"

DFS + Backtrace + Pruning

class Solution {
public:
    vector<vector<bool> > visited;
    bool checkPos(vector<vector<char> > &board, char c, int currX, int currY)
    {        
        if (currX < 0 || currX >= board[0].size()) return false;
        if (currY < 0 || currY >= board.size()) return false;
        if (visited[currY][currX]) return false;
        return board[currY][currX] == c;
    }
    bool _exist(vector<vector<char> > &board, string word, int currX, int currY)
    {
        if (word.length() == 0) return true;

        char c = word[0];
        visited[currY][currX] = true;

        if (word.length() == 1)
        {            
            return    (checkPos(board, c, currX - 1, currY)) ||
                    (checkPos(board, c, currX, currY - 1)) ||
                    (checkPos(board, c, currX + 1, currY)) ||
                    (checkPos(board, c, currX, currY + 1));
        }

        if (checkPos(board, c, currX - 1, currY))
        {
            bool bLeft = _exist(board, word.substr(1, word.length() - 1), currX - 1, currY);
            if (bLeft) return true;
            visited[currY][currX - 1] = false;
        }
        if (checkPos(board, c, currX, currY - 1))
        {
            bool bUp = _exist(board, word.substr(1, word.length() - 1), currX, currY - 1);
            if (bUp) return true;
            visited[currY - 1][currX] = false;
        }
        if (checkPos(board, c, currX + 1, currY))
        {
            bool bRight = _exist(board, word.substr(1, word.length() - 1), currX + 1, currY);
            if (bRight) return true;
            visited[currY][currX + 1] = false;
        }
        if (checkPos(board, c, currX, currY + 1))
        {
            bool bDown = _exist(board, word.substr(1, word.length() - 1), currX, currY + 1);
            if (bDown) return true;
            visited[currY + 1][currX] = false;
        }
        return false;
    }
    bool exist(vector<vector<char> > &board, string word) {
        int n = board.size(); if (n == 0) return false;
        int m = board[0].size(); if (m == 0) return false;
        
        visited.resize(n);
        for (int i = 0; i < n; i++)
        {
            visited[i].resize(m);
            std::fill(visited[i].begin(), visited[i].end(), false);
        }

        char c = word[0];
        for (int j = 0; j < n; j ++)
        for (int i = 0; i < m; i ++)
        {
            if (board[j][i] == c)
            {
                if (_exist(board, word.substr(1, word.length() - 1), i, j)) return true;
                visited[j][i] = false;
            }
        }
        return false;
    }
};