Solve three string/stack/backtracking problems

You are given three independent coding tasks (solve each one). Unless otherwise stated, implement a function with the described input/output.

Problem A (Monotonic stack)

Given an array heights of length n where heights[i] is the height of the i-th person standing in a line (from left to right). For each person i, compute how many people to the right are visible to person i.

A person j > i is visible to i if:

• For every k with i < k < j , heights[k] < min(heights[i], heights[j]) .

Return an array ans of length n where ans[i] is the number of visible people to the right of i.

Constraints (typical): 1 <= n <= 1e5, 1 <= heights[i] <= 1e9.

Problem B (Backtracking)

Given an integer n, generate all strings containing exactly n pairs of parentheses that are balanced.

Return the list of valid strings in any order.

Constraints (typical): 0 <= n <= 10.

Problem C (Greedy / Stack)

Given a string s consisting of lowercase letters and parentheses '(' and ')', remove the minimum number of parentheses so that the resulting string is a valid parentheses string (letters are always allowed and do not affect validity).

Return any valid result after minimum removals.

A parentheses string is valid if parentheses are balanced and properly nested.

Constraints (typical): 1 <= |s| <= 1e5.