Number of islands

class Solution {
public:
    void dfs(vector<vector<char>> &a,vector<vector<int>> &vis,int x,int y)
    {
        int r=a.size();
        int c=a[0].size();
        if(x>=r || x<0 || y>=c || y<0 || vis[x][y]==1 || a[x][y]=='0')
            return;
        vis[x][y]=1;
        dfs(a,vis,x-1,y);
        dfs(a,vis,x,y+1);
        dfs(a,vis,x,y-1);
        dfs(a,vis,x+1,y);
    }
    int numIslands(vector<vector<char>>& a) {
        int r=a.size();
        if(r==0)
            return 0;
        int c=a[0].size();
        if(c==0)
            return 0;
        vector<vector<int>> vis(r,vector<int>(c,0));
        int count=0;
        for(int i=0;i<r;i++)
        {
            for(int j=0;j<c;j++)
            {
                if(a[i][j]=='1' && vis[i][j]==0)
                {
                    dfs(a,vis,i,j);
                    count++;
                }
            }
        }
        return count;
    }
};