leetcode 419. Battleships in a Board

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:

...XXXXX...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?

我的方法是从左往右，从上往下遍历，遇到遍历过的或'.'就跳掉，遇到'X'就看看到底是往右还是往下发展的。如果是往右发展的，则j++，一直到结束，把battleship+1；如果是往下发展的，则temp=i，temp++，一直到结束，把battleship+1（注意不能i++，因为后几行的列还等着你遍历呢）

public int countBattleships(char[][] board) {int count=0;int xLength=board.length;//xLength是行数int yLength=board[0].length;//yLength是列数int[][] isGoneThroguh = new int[xLength][yLength];for(int i=0;i<xLength;i++){for(int j=0;j<yLength;j++){if(isGoneThroguh[i][j]==1){continue;}if(board[i][j]=='.'){isGoneThroguh[i][j]=1;continue;}if(board[i][j]=='X'){if(j+1<yLength&&board[i][j+1]=='X'){while(j<yLength&&board[i][j]=='X'){isGoneThroguh[i][j]=1;j++;}j--;count++;}else if(i+1<xLength&&board[i+1][j]=='X'){int temp=i;while(temp<xLength&&board[temp][j]=='X'){isGoneThroguh[temp][j]=1;temp++;}count++;}else{count++;}}}}return count;}public static void main(String[] args) {// TODO Auto-generated method stubBattleships_in_a_Board_419 b = new Battleships_in_a_Board_419();char[][] a=new char[][]{{'X','.','.','X'},{'.','.','.','X'},{'.','.','.','X'}};System.out.println(b.countBattleships(a));}}

然后大神的思路令我膜拜：

Going over all cells, we can count only those that are the "first" cell of the battleship. First cell will be defined as the most top-left cell. We can check for first cells by only counting cells that do not have an 'X' to the left and do not have an 'X' above them.

只需要数舰队中最上左的那只舰就好了。即你发现了一个'X'之后，如果它上面和左边没'X'的话，它就是最上左的舰，把count+1，反之则不是最上左的舰，直接跳过。

 public int countBattleships(char[][] board) {        int m = board.length;        if (m==0) return 0;        int n = board[0].length;                int count=0;                for (int i=0; i<m; i++) {            for (int j=0; j<n; j++) {                if (board[i][j] == '.') continue;                if (i > 0 && board[i-1][j] == 'X') continue;                if (j > 0 && board[i][j-1] == 'X') continue;                count++;            }        }                return count;    }

大神的解法多么简洁，多么漂亮。。。这真是智商的差别啊T_T！！