Swim in Rising Water in C++

You get a square grid of numbers, grid, with n rows and n columns. Each number is the height of one cell.

Return the earliest time at which you can swim from the top-left cell to the bottom-right cell.

Rain starts at time 0. At time t, the water is t deep everywhere. You can swim from a cell to a neighbour when both cells are at most t high. A neighbour is the next cell up, down, left or right.

Swimming takes no time, however far you go. You start in the top-left cell.

The grid is a vector of rows. Each row is a vector of numbers.

Example 1:

Input: grid = [
  [0, 5, 2],
  [6, 8, 3],
  [7, 4, 1]
]
Output: 5
Explanation: The neighbours of the top-left 0 are 5 and 6.
So you can't leave it before time 5.
At time 5, you can swim 0 → 5 → 2 → 3 → 1.

Example 2:

Input: grid = [
  [ 0, 20, 14,  8,  9],
  [ 1, 17,  7, 16, 10],
  [ 2, 23,  6, 22, 11],
  [ 3,  4,  5, 19, 12],
  [24, 15, 21, 18, 13]
]
Output: 14
Explanation: The best route goes down, then up, then down again:
0 → 1 → 2 → 3 → 4 → 5 → 6 → 7 → 14 → 8 → 9 → 10 → 11 → 12 → 13.
Its highest cell is 14, so you can swim it at time 14.
Every other route passes a cell that is 15 or higher.

Example 3:

Input: grid = [
  [3, 0],
  [1, 2]
]
Output: 3
Explanation: You start on a cell that is 3 high.
You can't swim away from it before time 3.

Constraints:

• n is from 1 to 50.
• The heights are the numbers from 0 to n² - 1.
• Each height appears exactly once.