Simulate bubble elimination and maximize common prefix

You are given two independent coding tasks (as in a multi-question OA). Solve each.

Task A — Grid “bubble/candy” elimination simulation

You are given an m x n grid board of integers.

Repeatedly apply the following steps until the board becomes stable (no more eliminations):

1. Mark for elimination: Any cell that is part of a horizontal or vertical run of 3 or more equal values is marked.
   • Marking is done based on the board state at the start of the round (i.e., all eliminations in a round happen simultaneously).
2. Eliminate: Set all marked cells to 0.
3. Gravity: For each column independently, let all non-zero values “fall” toward the bottom, preserving their relative order, and fill the remaining cells at the top with 0 (equivalently: “zeros float up”).

Output: Return the final stable grid.

Assumptions/constraints (typical OA):

• 1 <= m, n <= 50 (or similar)
• Cell values are non-negative integers; 0 represents empty.

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Task B — Maximum longest common prefix length from two lists

You are given two lists of strings: A and B.

Choose one string a from A and one string b from B. Let lcp(a, b) be the length of the longest common prefix between a and b.

Output: Return the maximum possible value of lcp(a, b) over all pairs (a, b).

Constraints (to rule out brute force):

• |A| and |B| can be large (e.g., up to 1e5 total strings)
• Total length of all strings can be large (e.g., up to 1e6)
• Lowercase English letters (or ASCII) for simplicity

Your solution should be efficient enough that checking all pairs is too slow.