Solve 2D array maze using recursion

Question

I'm writing a code to read in a txt file with a maze. Then solve that maze using recursion.

I have completely rewrote my code several time but it seem my code doesn't move from the starting point. I don't know why it's doing that.

public static void main(String[] args){
    //Read in maze file...

    //Find starting point
    int startRow = 0;
    int startColumn = 0;
    for(int i = 0; i <maze[0].length; i++)
    {
        if(maze[6][i] == 's')
        {
            startRow = 6;
            startColumn = i;
        }
    }

    if(solve(maze,startRow,startColumn))
    {
        System.out.println("Success");

        for(int r = 0; r < 7; r++)
           {
              for(int c = 0; c < 20; c++)
              {
                 System.out.print(maze[r][c]);
              }
              System.out.println();
           }
    }
    else
    {
        System.out.println("Fail");

        for(int r = 0; r < 7; r++)
           {
              for(int c = 0; c < 20; c++)
              {
                 System.out.print(maze[r][c]);
              }
              System.out.println();
           }
    }
}

public static boolean solve(char[][] maze, int row, int column)
{
      boolean success = false;
      if(valid(maze, row, column))
      {
         maze[row][column] = 'v';  //mark as visited

         if (maze[row][column] == 'f') //check for finish
            success = true;
         else
         {
            success = solve(maze, row - 1, column);  //north
            if(!success)
               success = solve(maze, row, column + 1);  //west
            if(!success)
               success = solve(maze, row, column - 1);  //east
            if(!success)
               success = solve(maze, row + 1, column);  //south
         }
         if(success)  //mark as path
            maze[row][column] = 'p';
      }
      return success;
 }
public static boolean valid(char[][] maze, int row, int column)
{
    boolean a = false;
    if(row >= 0 && row < maze.length && column >= 0 && column < maze[0].length)
         if (maze[row][column] == ' ')
            a = true;
      return a;
}

}

I'm using a 7x20 text file for testing:

xxxxxxxxxxxxxxxxxxfx
x     x       xxxx x
x xxxxx xxxxx   xx x
x xxxxx xxxxxxx xx x
x            xx xx x
x xxxxxxxxxx xx    x 
xxxxxxxxxxxxsxxxxxxx

'x' = wall

's' = start

'f' = finish

My output:

Fail
xxxxxxxxxxxxxxxxxxfx
x     x       xxxx x
x xxxxx xxxxx   xx x
x xxxxx xxxxxxx xx x
x            xx xx x
x xxxxxxxxxx xx    x
xxxxxxxxxxxxsxxxxxxx

Answer 1

Your search never gets started because your valid method reports the starting point, s as invalid.

The quick fix is to change:

if (maze[row][column] == ' ')
  a = true;

to

if (maze[row][column] == ' ' || maze[row][column] == 's')
  a = true;

The other problem is that you overwrite the finish cell as visited before checking for it:

maze[row][column] = 'v';  //mark as visited

if (maze[row][column] == 'f') //check for finish
  success = true;

You need to restructure your solve method to look something like this:

public static boolean solve(char[][] maze, int row, int column)
{
      boolean success = false;

      if (maze[row][column] == 'f') //check for finish
        success = true;
      else if(valid(maze, row, column))
      {
         maze[row][column] = 'v';  //mark as visited

          success = solve(maze, row - 1, column);  //north
          if(!success)
             success = solve(maze, row, column + 1);  //west
          if(!success)
             success = solve(maze, row, column - 1);  //east
          if(!success)
             success = solve(maze, row + 1, column);  //south

         if(success)  //mark as path
            maze[row][column] = 'p';
      }
      return success;
}

With these changes your code works beautifully:

Success
xxxxxxxxxxxxxxxxxxfx
x     xpppppppxxxxpx
x xxxxxpxxxxxpppxxpx
x xxxxxpxxxxxxxpxxpx
x      ppppppxxpxxpx
x xxxxxxxxxxpxxppppx
xxxxxxxxxxxxpxxxxxxx