LeetCode 286. Walls and Gates（墙与门）

原题网址：https://leetcode.com/problems/walls-and-gates/

You are given a m x n 2D grid initialized with these three possible values.

1. -1 - A wall or an obstacle.
2. 0 - A gate.
3. INF - Infinity means an empty room. We use the value 231 - 1 = 2147483647 to represent INF as you may assume that the distance to a gate is less than2147483647.

Fill each empty room with the distance to its nearest gate. If it is impossible to reach a gate, it should be filled with INF.

For example, given the 2D grid:

INF  -1  0  INF
INF INF INF  -1
INF  -1 INF  -1
  0  -1 INF INF

After running your function, the 2D grid should be:

  3  -1   0   1
  2   2   1  -1
  1  -1   2  -1
  0  -1   3   4

方法一：深度优先搜索。

public class Solution {
    private void tag(int[][] rooms, int row, int col, int dist) {
        if (row < 0 || row >= rooms.length || col < 0 || col >= rooms[row].length) return;
        if (rooms[row][col] == -1 || rooms[row][col] < dist) return;
        rooms[row][col] = dist;
        tag(rooms, row, col-1, dist+1);
        tag(rooms, row, col+1, dist+1);
        tag(rooms, row-1, col, dist+1);
        tag(rooms, row+1, col, dist+1);
    }
    public void wallsAndGates(int[][] rooms) {
        for(int row = 0; row < rooms.length; row ++) {
            for(int col = 0; col < rooms[row].length; col ++) {
                if (rooms[row][col] == 0) tag(rooms, row, col, 0);
            }
        }
    }
}

方法二：广度优先搜索

public class Solution {
    public void wallsAndGates(int[][] rooms) {
        Position start = new Position(0,0,0);
        Position tail = start;
        for(int row=0; row= 0 && current.row < rooms.length && current.col >= 0 && current.col < rooms[current.row].length && rooms[current.row][current.col] >= current.dist) {
                rooms[current.row][current.col] = current.dist;
                tail.next = new Position(current.row, current.col-1, current.dist+1);
                tail = tail.next;
                tail.next = new Position(current.row, current.col+1, current.dist+1);
                tail = tail.next;
                tail.next = new Position(current.row-1, current.col, current.dist+1);
                tail = tail.next;
                tail.next = new Position(current.row+1, current.col, current.dist+1);
                tail = tail.next;
            }
            current = current.next;
        }
    }
}
class Position {
    int row, col, dist;
    Position next;
    Position(int row, int col, int dist) {
        this.row = row;
        this.col = col;
        this.dist = dist;
    }
}