419. Battleships in a Board
Đề Bài
Given an m x n matrix board where each cell is a battleship 'X' or empty '.', return the number of the battleships on board.
Battleships can only be placed horizontally or vertically on board. In other words, they can only be made of the shape 1 x k (1 row, k columns) or k x 1 (k rows, 1 column), where k can be of any size. At least one horizontal or vertical cell separates between two battleships (i.e., there are no adjacent battleships).
Example 1:
Input: board = [["X",".",".","X"],[".",".",".","X"],[".",".",".","X"]] Output: 2
Example 2:
Input: board = [["."]] Output: 0
Constraints:
m == board.lengthn == board[i].length1 <= m, n <= 200board[i][j]is either'.'or'X'.
Follow up: Could you do it in one-pass, using only O(1) extra memory and without modifying the values board?
Thuật Toán & Kỹ Thuật
⏱️ Thời gian
O(V+E)
💾 Không gian
O(V)
Lời Giải
Python
0419-battleships-in-a-board.py
class Solution:
def countBattleships(self, board: List[List[str]]) -> int:
DIR = [-1, 0, 1, 0, -1]
n = len(board)
m = len(board[0])
def dfs(x, y):
if not (0 <= x and x < n and 0 <= y and y < m and board[x][y] != '.'):
return
board[x][y] = '.'
for i in range(4):
dfs(x + DIR[i], y + DIR[i + 1])
cnt = 0
for i in range(n):
for j in range(m):
if board[i][j] == '.':
continue
cnt += 1
dfs(i, j)
return cnt