Solve interval merge and string dedup problems

You are given three coding questions from a phone screen. Solve each independently.

1) Merge overlapping intervals

Input: A list of intervals intervals, where each interval is [start, end] and start <= end.

Task: Merge all overlapping intervals and return a list of non-overlapping intervals that cover the same ranges.

Output: The merged intervals, typically sorted by start.

Example:

• Input: [[1,3],[2,6],[8,10],[15,18]]
• Output: [[1,6],[8,10],[15,18]]

Constraints (assume):

• 1 <= n <= 10^5
• Interval endpoints fit in 32-bit signed integers.

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2) Remove adjacent duplicates (pair removal)

Input: A string s.

Task: Repeatedly remove adjacent duplicate characters when they appear as a pair (e.g., remove "aa"), until no more such pairs exist.

Output: The final resulting string.

Example:

• Input: "abbaca"
• Process: abbaca -> aaca -> ca
• Output: "ca"

Constraints (assume):

• 1 <= |s| <= 10^5
• Characters can be treated as ASCII letters/digits.

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3) Remove adjacent duplicates in groups of size k

Input: A string s and an integer k >= 2.

Task: Repeatedly remove any group of k identical characters that are adjacent. Continue until no such group exists.

Output: The final resulting string.

Example:

• Input: s = "deeedbbcccbdaa", k = 3
• Output: "aa"

Constraints (assume):

• 1 <= |s| <= 10^5
• 2 <= k <= 10^5
• Characters can include letters and digits.