LeetCode Battleships in a Board

本文介绍了一道LeetCode上的经典题目“战舰游戏”的解决方案。该题要求在二维棋盘中计数战舰数量,战舰由字符'X'表示且仅能水平或垂直放置。文章提供了一个一次遍历的高效解法,并附带了Java实现代码。

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原题链接在这里:https://leetcode.com/problems/battleships-in-a-board/#/description

题目:

Given an 2D board, count how many battleships are in it. The battleships are represented with 'X's, empty slots are represented with '.'s. You may assume the following rules:

  • You receive a valid board, made of only battleships or empty slots.
  • Battleships can only be placed horizontally or vertically. In other words, they can only be made of the shape 1xN (1 row, N columns) or Nx1 (N rows, 1 column), where N can be of any size.
  • At least one horizontal or vertical cell separates between two battleships - there are no adjacent battleships.

Example:

X..X
...X
...X

In the above board there are 2 battleships.

Invalid Example:

...X
XXXX
...X

This is an invalid board that you will not receive - as battleships will always have a cell separating between them.

Follow up:
Could you do it in one-pass, using only O(1) extra memory and without modifying the value of the board?

题解:

无论是1*N 还是 N*1型的battleship, 都以左上角的X来标志. 只有遇到X, 并且左边或者上边没有连着的X时才算新的battleship.

Time Complexity: O(mn), m = board.length, n = board[0].length.

Space: O(1).

AC Java:

 1 public class Solution {
 2     public int countBattleships(char[][] board) {
 3         if(board == null || board.length == 0 || board[0].length == 0){
 4             return 0;
 5         }
 6         
 7         int res = 0;
 8         for(int i = 0; i<board.length; i++){
 9             for(int j = 0; j<board[0].length; j++){
10                 if(board[i][j] == '.'){
11                     continue;
12                 }
13                 if(i>0 && board[i-1][j]=='X'){
14                     continue;
15                 }
16                 if(j>0 && board[i][j-1]=='X'){
17                     continue;
18                 }
19                 res++;
20             }
21         }
22         return res;
23     }
24 }

 

转载于:https://www.cnblogs.com/Dylan-Java-NYC/p/6751562.html

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