Rising tide crossing

For each trip request, find the lowest tide level at which a flat boat can make the trip across a marsh.

A marsh is a grid of cells. The string rows[r] describes row r, and the letter rows[r][c] gives the ground height of that cell: "a" is height 0, "b" is 1, and so on up to "z" which is 25. When the tide is at level L, every cell whose height is at most L is under water and a boat can sit in it; every other cell is dry ground the boat cannot enter. The tide level is a whole number, and the boat moves between cells that share an edge (up, down, left, right; no diagonals) as long as every cell it enters is under water.

A trip request [r1, c1, r2, c2] asks for the smallest tide level L (a whole number from 0 to 25) at which a boat can get from cell (r1, c1) to cell (r2, c2). Both the starting cell and the destination cell must themselves be under water, so a trip that starts and ends in the same cell needs a level at least as high as that cell. Every cell is under water at level 25, so an answer always exists.

Input:

• rows: a non-empty list of equal-length strings of lowercase letters.
• trips: a list of [r1, c1, r2, c2] requests (possibly empty). All coordinates are inside the grid.

Output: a list of integers, one per request in the order given: the lowest tide level that makes the trip possible.

There can be hundreds of requests on a large marsh, so searching the marsh again for every request will be too slow.