Implement list and grid transformation tasks
Company: eBay
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Technical Screen
You are given three independent coding tasks (as in an online assessment). Implement each as a separate function.
## 1) Uppercase–lowercase difference
**Input:** A list of characters `chars` (each element is a 1-length string).
**Output:** An integer equal to:
\[
(\#\text{uppercase letters}) - (\#\text{lowercase letters})
\]
**Notes:**
- Use typical letter checks (e.g., `isupper()`, `islower()` semantics).
- Non-letters (digits, punctuation, whitespace, etc.) do not affect the result.
---
## 2) Repartition numbers into two lists, then merge
**Input:** An integer array `nums`.
**Process:** Iterate `nums` from left to right and build two lists `A` and `B`.
For each number `x`:
1. Let `maxA` be the maximum value currently in `A` (or \(-\infty\) if `A` is empty). Let `maxB` be the maximum value currently in `B` (or \(-\infty\) if `B` is empty).
2. If `maxA > x` **or** `maxB > x`, append `x` to the list whose current maximum is larger:
- if `maxA >= maxB`, append to `A`; else append to `B`.
3. Otherwise (i.e., `x` is at least as large as both current maxima), append `x` to the **shorter** list (if tied, append to `A`).
**Output:** Return the concatenation `A + B`.
---
## 3) Balloon explosion in an n×n grid with gravity
**Input:** An `n × n` integer matrix `grid` where each positive integer represents a balloon color/label, and `0` represents an empty cell.
**Explosion rule (4-neighborhood):** For any cell `(r, c)` with value `v > 0`, count how many of its **up/down/left/right** neighbors also have value `v`. If that count is **≥ 2**, then the balloon at `(r, c)` explodes.
- All explosions in a round happen **simultaneously** (decide based on the grid state at the start of the round).
- Exploded cells become `0`.
**Gravity rule:** After explosions, apply gravity column by column: within each column, all non-zero values fall downward to fill empty spaces, preserving their relative order (stable fall).
**Repeat:** Repeat “explosion round → gravity” until a full round causes **no explosions**.
**Output:** Return the final stabilized grid.
**Clarifications:**
- Only 4-direction adjacency counts (no diagonals).
- Edge cells have fewer neighbors.
Quick Answer: This question evaluates skills in character classification, array and list manipulation, greedy repartitioning logic, and iterative grid simulation with simultaneous state updates and gravity effects.