Leetcode 200. Number of Islands

class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        if(grid.size() == 0) {
            return 0;
        }
        int islands = 0;
        int rows = grid.size();
        int columns = grid[0].size();
        for(int i = 0; i < rows; i++) {
            for(int j = 0; j < columns; j++) {
                if(grid[i][j] == '1') {
                    islands++;
                    removeIsland(grid, i, j, rows, columns);
                }
            }
        }
        return islands;
    }

private:
    void removeIsland(vector<vector<char>>& grid, int i, int j, int& rows, int& columns) {
        if(i < 0 || i == rows || j < 0 || j == columns || grid[i][j] == '0') {
            return;
        }
        grid[i][j] = '0';
        removeIsland(grid, i + 1, j, rows, columns);
        removeIsland(grid, i - 1, j, rows, columns);
        removeIsland(grid, i, j + 1, rows, columns);
        removeIsland(grid, i, j - 1, rows, columns);
    }
};