 # 2849. Determine if a Cell Is Reachable at a Given Time

Updated: 2023-09-10 LeetCode problem 2849

## Problem Statement

You are given starting coordinates (sx, sy) and final coordinates (fx, fy) on a 2D infinite grid. Starting from the initial position, you can move to any of the 8 adjacent cells in one second. The challenge is to determine if you can reach the final cell (fx, fy) in exactly t seconds.

## Naive Solution

A naive approach would be to use a recursive function to navigate the grid from the starting point (sx, sy) and attempt to reach the target (fx, fy) in t seconds. This approach will check every possible path, which can be very inefficient, especially for larger grids and values of t.

## Hints & Tips

• Understand that not all paths are equal. Moving diagonally is more efficient than moving in a straight line if both x and y distances are positive.
• Check the constraints. If you cannot reach the destination within the given time t, there’s no need to try any further.

## Approach

The idea is to calculate the shortest possible time to reach the target and then compare it to the given time t. If the starting point and the ending point are the same, we can always return to the same position unless the time is exactly 1.

## Steps

1. Calculate the difference in x (diff_x) and y (diff_y) coordinates.
2. For the minimum of diff_x and diff_y, move diagonally. This reduces both x and y distances by 1 in a single step.
3. Move horizontally or vertically for the absolute difference between diff_x and diff_y.
4. If (diff_x + diff_y) is less than or equal to t, and (t - (diff_x + diff_y)) is an even number or zero, return True, otherwise return False.

## Solution

def isReachableAtTime(sx: int, sy: int, fx: int, fy: int, t: int) -> bool:
diff_x = abs(fx - sx)           # Calculating the differences in x
diff_y = abs(fy - sy)           # and y coordinates

if diff_x == 0 and diff_y == 0: # If both starting and ending points are same
return t != 1

if diff_x <= t and diff_y <= t: # Check if we can reach the target within given time
return True

return False