Find Column Index of Left-Most 1 in Binary Matrix using Python

Suppose we have a 2D binary matrix. Here each row is sorted in ascending order with 0s coming before 1s, we have to find the leftmost column index with the value of 1. If there's no such result, return -1.

So, if the input is like

then the output will be 2, as the second column has left most 1 in entire matrix.

To solve this, we will follow these steps:

• if matrix is empty, then

  • return -1

• N := row count of matrix

• M := column count of matrix

• i := 0, j := M - 1

• leftmost := -1

• while i < N and j >= 0, do

  • if matrix[i, j] is same as 0, then

    • i := i + 1

  • otherwise,

    • leftmost := j

    • j := j - 1

• return leftmost

Example

 Live Demo

class Solution:
   def solve(self, matrix):
      if not matrix or not matrix[0]:
         return -1

      N = len(matrix)
      M = len(matrix[0])

      i = 0
      j = M - 1

      leftmost = -1

      while i < N and j >= 0:
         if matrix[i][j] == 0:
            i += 1
         else:
            leftmost = j
            j -= 1

      return leftmost

ob = Solution()
matrix = [
   [0, 0, 0, 1],
   [0, 0, 1, 1],
   [0, 0, 1, 1],
   [0, 0, 1, 0]
]
print(ob.solve(matrix))

Input

[
[0, 0, 0, 1],
[0, 0, 1, 1],
[0, 0, 1, 1],
[0, 0, 1, 0] ]

Output

2