leetcode Number of Islands

public class Solution {
public int numIslands(char[][] grid) {
int count=0;
for(int i=0;i<grid.length;i++)
{
for(int j=0;j<grid[0].length;j++)
{
if(grid[i][j]=='1')
{
count++;
dfs(grid, i, j);
}

}
}
return count;
}

public void dfs(char[][]grid,int rowIndex,int columIndex)
{
if(rowIndex<0||rowIndex>grid.length-1)
{
return;
}

if(columIndex<0||columIndex>grid[0].length-1)
{
return;
}

if(grid[rowIndex][columIndex]=='0')
{
return;
}

grid[rowIndex][columIndex]='0';
dfs(grid,rowIndex-1,columIndex);
dfs(grid,rowIndex+1,columIndex);
dfs(grid, rowIndex, columIndex-1);
dfs(grid, rowIndex, columIndex+1);
return;
}
}