Find the Longest Straight Run of One Color

Problem Statement

Given an m x n 2D grid of integers representing colors, find the length of the longest straight contiguous line segment of cells having the exact same color.

A line segment can run in any of the four straight directions:

• Horizontal (left to right)
• Vertical (top to bottom)
• Main diagonal (top-left to bottom-right, +1 row, +1 col)
• Anti-diagonal (top-right to bottom-left, +1 row, -1 col)

Return the maximum number of cells in any such valid run. If the grid is empty, return 0.

Function Signature

def longest_straight_run(grid: list[list[int]]) -> int:
    pass

Input & Output Format

• Input: grid, a list of lists of integers, where grid[r][c] represents the color of the cell at row r and column c.
• Output: An integer representing the length of the longest straight sequence of identical colors.

Constraints

• 0 <= m, n <= 1000
• 0 <= m * n <= 10^6
• -10^9 <= grid[r][c] <= 10^9

Examples

Example 1

Input: grid = [
  [1, 2, 1, 3],
  [2, 1, 3, 1],
  [1, 3, 1, 1],
  [3, 1, 2, 1]
]
Output: 4
Explanation: The main diagonal starting at (0, 0) and ending at (3, 3) has values [1, 1, 1, 1], giving a length of 4.

Example 2

Input: grid = [
  [5, 5, 5, 2],
  [1, 5, 2, 2],
  [3, 5, 2, 2]
]
Output: 3
Explanation: 
- The top row has three consecutive 5s: (0, 0), (0, 1), (0, 2).
- The second column has three consecutive 5s: (0, 1), (1, 1), (2, 1).
Both yield a maximum length of 3.

Edge Cases

• Empty grid (m = 0 or n = 0): should return 0.
• Grid with a single element (1 x 1): should return 1.
• All elements are different: should return 1.
• All elements are identical: should return max(m, n, min(m, n)).