531. Lonely Pixel I

• Difficulty: Medium

• Topics: Array, Depth-first Search

• Similar Questions:

  • Lonely Pixel II

Problem:

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:

Solutions:

class Solution {
public:
    int findLonelyPixel(vector<vector<char>>& picture) {
        int m = picture.size();
        if (m == 0) return 0;
        int n = picture[0].size();
        if (n == 0) return 0;

        int ret = 0;

        for (int row = 0; row < m; ++row) {
            int blackIdx = -1;
            int blackCount = 0;
            for (int col = 0; col < n; ++col) {
                if (picture[row][col] == 'B') {
                    blackIdx = col;
                    ++blackCount;
                    if (blackCount > 1) break;
                }
            }

            if (blackCount == 1) {
                int i;
                for (i = 0; i < m; ++i) {
                    if (i == row)   continue;
                    if (picture[i][blackIdx] == 'B')    break;
                }
                if (i == m) ++ret;
            }        
        }

        return ret;
    }
};