Remove adjacent duplicates and handle tree input

You are asked three related coding questions.

1) Remove adjacent duplicates (repeatedly)

Given a string s (lowercase English letters), repeatedly remove any pair of adjacent equal characters until no such pair exists.

• Input: s
• Output: the final string after all removals

Example:

• s = "abbaca" → remove bb → aaca → remove aa → ca

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

2) Follow-up: remove groups of size k

Given a string s and an integer k, repeatedly remove any group of exactly k adjacent equal characters until no more removals are possible.

• Input: s , k
• Output: final string

Example:

• s = "deeedbbcccbdaa", k = 3 → output "aa"

Constraints (typical): 1 <= |s| <= 1e5, 2 <= k <= 1e4.

3) Tree problem, but input is an array (must build the tree)

You are given a binary tree in level-order array form (not TreeNode objects). null indicates a missing child. Build the binary tree and compute:

The sum of all left leaf nodes (a left leaf is a leaf node that is the left child of its parent).

• Input: array arr of integers and null s, in level-order
• Output: integer sum

Example:

• arr = [3,9,20,null,null,15,7] → left leaves are 9 and 15 → output 24

Notes: You must construct the tree from the array representation before computing the answer.