LeetCode 531. Longly Pixel I （孤独的像素之一） $

Given a picture consisting of black and white pixels, find the number of black lonely pixels.

The picture is represented by a 2D char array consisting of 'B' and 'W', which means black and white pixels respectively.

A black lonely pixel is character 'B' that located at a specific position where the same row and same column don't have any other black pixels.

Example:

Input: 
[['W', 'W', 'B'],
 ['W', 'B', 'W'],
 ['B', 'W', 'W']]

Output: 3
Explanation: All the three 'B's are black lonely pixels.

Note:

1. The range of width and height of the input 2D array is [1,500].

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题目标签：Array

　　题目给了我们一个2d picture array，让我们找出有几个孤独的像素B。孤独的像素B的行和列中只有自己一个像素。

　　可以建立2个 array， 分别是 row 和 col， 它们的size 就是picture里的行和列的size。

　　第一次遍历picture：如果是B，就把B的row 和 column 在 row array 和 col array 里对应位置 加1。目的是为了记录每一行和每一列里有多少个B。

　　第二次遍历picture：如果是B，就到对应的 row  和 col array 里， 如果 row 和 col 的值都是1 的话，说明这一个B 是 row 这一行里， 和 col 这一列里唯一的B。累计计数到res。

Java Solution:

Runtime beats 80.41% 

完成日期：09/24/2017

关键词：Array

关键点：设立row array & col array 记录每一行每一列里有多少个B；孤单像素所在的行和列只有它自己

class Solution 
{
    public int findLonelyPixel(char[][] picture) 
    {
        int n = picture.length;
        int m = picture[0].length;
        // create two array for counting B
        int[] row = new int[n];
        int[] col = new int[m];
        
        int res = 0; // counts longly black
        
        // 1st iteration, if find a B, increase its row and col number in two array
        for(int i=0; i<n; i++)
        {
            for(int j=0; j<m; j++)
            {
                if(picture[i][j] == 'B')
                {
                    row[i]++;
                    col[j]++;
                }
            }
        }
        
        
        // 2nd iteration, if find a B, check its row and col are both 1, meaning lonly B
        for(int i=0; i<n; i++)
            for(int j=0; j<m; j++)
                if(picture[i][j] == 'B' && row[i] == 1 && col[j] == 1)
                    res++;
                    
        return res;
    }
}

参考资料：

https://discuss.leetcode.com/topic/81680/java-o-nm-time-with-o-n-m-space-and-o-1-space-solutions

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