The Maze.java

M
1534262484
tags:  DFS, BFS

#### BFS
- BFS on coordinates
- always attempt to move to end of border
- use boolean[][] visited to alingn with BFS solution in Maze II, III, where it uses Node[][] to store state on each item.

```
/*
There is a ball in a maze with empty spaces and walls. The ball can go through empty spaces by rolling up, down, left or right, but it won't stop rolling until hitting a wall. When the ball stops, it could choose the next direction.

Given the ball's start position, the destination and the maze, determine whether the ball could stop at the destination.

The maze is represented by a binary 2D array. 1 means the wall and 0 means the empty space. You may assume that the borders of the maze are all walls. The start and destination coordinates are represented by row and column indexes.

Example 1

Input 1: a maze represented by a 2D array

0 0 1 0 0
0 0 0 0 0
0 0 0 1 0
1 1 0 1 1
0 0 0 0 0

Input 2: start coordinate (rowStart, colStart) = (0, 4)
Input 3: destination coordinate (rowDest, colDest) = (4, 4)

Output: true
Explanation: One possible way is : left -> down -> left -> down -> right -> down -> right.

Example 2

Input 1: a maze represented by a 2D array

0 0 1 0 0
0 0 0 0 0
0 0 0 1 0
1 1 0 1 1
0 0 0 0 0

Input 2: start coordinate (rowStart, colStart) = (0, 4)
Input 3: destination coordinate (rowDest, colDest) = (3, 2)

Output: false
Explanation: There is no way for the ball to stop at the destination.

Note:
There is only one ball and one destination in the maze.
Both the ball and the destination exist on an empty space, and they will not be at the same position initially.
The given maze does not contain border (like the red rectangle in the example pictures), but you could assume the border of the maze are all walls.
The maze contains at least 2 empty spaces, and both the width and height of the maze won't exceed 100.

*/
// BFS with node
class Solution {
    int[] dx = {1, -1, 0, 0};
    int[] dy = {0, 0, 1, -1};
    public boolean hasPath(int[][] maze, int[] start, int[] dest) {
        int m = maze.length, n = maze[0].length;
        boolean[][] visited = new boolean[m][n];
        Queue<Node> queue = new LinkedList<>();
        queue.offer(new Node(start[0], start[1]));
        visited[start[0]][start[1]] = true;
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                Node node = queue.poll();
                for (int j = 0; j < 4; j++) {
                    int x = node.x, y = node.y;
                    while (x >= 0 && x < m && y >= 0 && y < n && maze[x][y] == 0) {
                        x+=dx[j];
                        y+=dy[j];
                    }
                    x-=dx[j];
                    y-=dy[j];
                    if (dest[0] == x && dest[1] == y) return true;
                    if (visited[x][y]) continue;
                    visited[x][y] = true;
                    queue.offer(new Node(x, y));
                }
            }
        }
        return false;
    }
    
    class Node {
        int x, y;
        public Node(int x, int y) {
            this.x = x;
            this.y = y;
        }
    }
}

// BFS with object[][]
class Solution {
    int[] dx = {1, -1, 0, 0};
    int[] dy = {0, 0, 1, -1};
    public boolean hasPath(int[][] maze, int[] start, int[] dest) {
        int m = maze.length, n = maze[0].length;
        boolean[][] visited = new boolean[m][n];
        Queue<int[]> queue = new LinkedList<>();
        queue.offer(start);
        visited[start[0]][start[1]] = true;
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                int[] pos = queue.poll();
                for (int j = 0; j < 4; j++) {
                    int x = pos[0], y = pos[1];
                    while (x >= 0 && x < m && y >= 0 && y < n && maze[x][y] == 0) {
                        x+=dx[j];
                        y+=dy[j];
                    }
                    x-=dx[j];
                    y-=dy[j];
                    if (dest[0] == x && dest[1] == y) return true;
                    if (visited[x][y]) continue;
                    visited[x][y] = true;
                    queue.offer(new int[] {x, y});
                }
            }
        }
        return false;
    }
}

// BFS
class Solution {
    int[] dx = {1, -1, 0, 0};
    int[] dy = {0, 0, 1, -1};
    public boolean hasPath(int[][] maze, int[] start, int[] dest) {
        int n = maze.length;
        int m = maze[0].length;
        Set<String> visited = new HashSet<>();
        Queue<int[]> queue = new LinkedList<>();
        queue.offer(start);
        visited.add(start[0] + "@" + start[1]);
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                int[] pos = queue.poll();
                if (verify(pos, dest)) return true;
                for (int j = 0; j < 4; j++) {
                    int x = pos[0], y = pos[1];
                    while (x >= 0 && x < n && y >= 0 && y < m && maze[x][y] == 0) {
                        x+=dx[j];
                        y+=dy[j];
                    }
                    x-=dx[j];
                    y-=dy[j];
                    String key = x + "@" + y;
                    if (!visited.contains(key)) {
                        visited.add(key);
                        queue.offer(new int[] {x, y});
                    }
                }
            }
        }
        return false;
    }
    
    private boolean verify(int[] pos, int[] dest) {
        return pos[0] == dest[0] && pos[1] == dest[1];
    }
}
```