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1582. Special Positions in a Binary Matrix

Difficulty Topics

Description

Given an m x n binary matrix mat, return the number of special positions in mat.

A position (i, j) is called special if mat[i][j] == 1 and all other elements in row i and column j are 0 (rows and columns are 0-indexed).

 

Example 1:

Input: mat = [[1,0,0],[0,0,1],[1,0,0]]
Output: 1
Explanation: (1, 2) is a special position because mat[1][2] == 1 and all other elements in row 1 and column 2 are 0.

Example 2:

Input: mat = [[1,0,0],[0,1,0],[0,0,1]]
Output: 3
Explanation: (0, 0), (1, 1) and (2, 2) are special positions.

 

Constraints:

  • m == mat.length
  • n == mat[i].length
  • 1 <= m, n <= 100
  • mat[i][j] is either 0 or 1.

Solution

special-positions-in-a-binary-matrix.py
class Solution:
    def numSpecial(self, mat: List[List[int]]) -> int:
        r = len(mat)
        c = len(mat[0])
        row = [0] * r
        col = [0] * c

        for i in range(r):
            for j in range(c):
                if mat[i][j]:
                    row[i] += 1
                    col[j] += 1

        res = 0

        for i in range(r):
            for j in range(c):
                if mat[i][j]:
                    if row[i] == 1 and col[j] == 1:
                        res += 1

        return res