Solve OA tasks on string, grid path, subarrays

You are given three independent coding tasks.

Task 1: Reverse the middle if ends are vowels

Given a string s (ASCII letters), if both the first and last characters are vowels (a,e,i,o,u case-insensitive), return a new string where:

• the first and last characters stay in place, and
• the substring strictly between them is reversed.

Otherwise, return s unchanged.

Examples

• "apple" → first= a (vowel), last= e (vowel) ⇒ reverse middle "ppl" → "alppe"
• "code" → first= c not vowel ⇒ "code"

Edge cases

• If len(s) <= 2 , there is no “middle”; return s .

Task 2: Maximize value along a constrained grid path

You are given an m x n grid of characters. Each cell contains either:

• a digit '0'..'9' , or
• an operator '+' or '-' .

You start at the top-left cell (0,0) and must reach the bottom-right cell (m-1,n-1) by moving only:

• right , or
• down .

As you traverse cells, you concatenate their characters to form an arithmetic expression, which is evaluated left-to-right (i.e., no operator precedence beyond sequential evaluation).

Validity assumption: Any path considered must form a valid expression of the form digit (op digit)* (alternating digit/operator), and it must start and end with a digit.

Direction-change constraint: Along the path, you are allowed to change direction at most once (i.e., the move sequence is either all rights then all downs, or all downs then all rights).

Goal: Return the maximum value achievable among all valid paths satisfying the movement constraint.

Task 3: Count alternating odd/even subarrays

Given an integer array nums, count how many contiguous subarrays have strictly alternating parity (odd/even/odd/even... or even/odd/even/odd...).

A subarray of length 1 always counts.

Example

• nums = [1, 2, 4]
  • Alternating subarrays: [1] , [2] , [4] , [1,2] , but [2,4] is not alternating (even, even), and [1,2,4] is not alternating.
  • Answer = 4

Performance requirement: Design an algorithm faster than brute force (e.g., O(n)).