Count occurences of a given word in a 2-d array

Find the number of occurrences of a given search word in a 2d-Array of characters where the word can go up, down, left, right and around 90 degree bends.

• Time : O(nm * |target|)
• Space : O(|target|)

class Solution {
    int res, n, m;
    void floodfill(vector<vector<char>> &mat, string& t, int p, int y, int x) {
        if(p == t.length()) res++;
        else {
            if(0 <= y and y < n and 0 <= x and x < m and mat[y][x] == t[p]) {
                mat[y][x] = '#';
                
                floodfill(mat, t, p + 1, y - 1, x);
                floodfill(mat, t, p + 1, y + 1, x);
                floodfill(mat, t, p + 1, y, x + 1);
                floodfill(mat, t, p + 1, y, x - 1);
                
                mat[y][x] = t[p];
            }
        }
    }
public:
    int findOccurrence(vector<vector<char>> &mat, string target) {
        res = 0, n = mat.size(), m = mat[0].size();
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < m; j++) {
                floodfill(mat, target, 0, i, j);
            }
        }
        return res / 4;
    }
};