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This question is commonly asked for Software Engineer candidates at Meta during technical interviews.

Validate parentheses and find k-th largest

Q: Validate parentheses and find k-th largest

This pair of problems evaluates mastery of fundamental data structures and algorithmic techniques: Problem A tests understanding of stack-based parsing and bracket-matching invariants for validating nested delimiters, while Problem B assesses proficiency with selection algorithms and heap-based priority queues for finding the k-th largest element.

Problem A — Validate parentheses

Given a string s containing only the characters '(', ')', '[', ']', '{', '}', determine whether the input string is valid.

A string is valid if:

Open brackets are closed by the same type of brackets.
Open brackets are closed in the correct order .
Every closing bracket has a corresponding previously-opened bracket.

Input: s (string)

Output: true if s is valid, otherwise false.

Example

s = "()[]{}" → true
s = "([)]" → false

Constraints (typical): 1 <= |s| <= 10^4

Problem B — k-th largest element using a heap

Given an integer array nums and an integer k, return the k-th largest element in the array.

Notes:

“k-th largest” refers to the element that would appear at index k-1 if the array were sorted in descending order.
Duplicates count as separate elements.

Input: nums (array of integers), k (integer)

Output: an integer, the k-th largest element.

Example

nums = [3,2,1,5,6,4], k = 2 → 5

Constraints (typical):

1 <= n <= 10^5
1 <= k <= n
nums[i] fits in 32-bit signed int

Problem A — Validate parentheses

Given a string s containing only the characters '(', ')', '[', ']', '{', '}', determine whether the input string is valid.

A string is valid if:

Open brackets are closed by the same type of brackets.
Open brackets are closed in the correct order .
Every closing bracket has a corresponding previously-opened bracket.

Input: s (string)

Output: true if s is valid, otherwise false.

Example

s = "()[]{}" → true
s = "([)]" → false

Constraints (typical): 1 <= |s| <= 10^4

Problem B — k-th largest element using a heap

Given an integer array nums and an integer k, return the k-th largest element in the array.

Notes:

“k-th largest” refers to the element that would appear at index k-1 if the array were sorted in descending order.
Duplicates count as separate elements.

Input: nums (array of integers), k (integer)

Output: an integer, the k-th largest element.

Example

nums = [3,2,1,5,6,4], k = 2 → 5

Constraints (typical):

1 <= n <= 10^5
1 <= k <= n
nums[i] fits in 32-bit signed int

Validate parentheses and find k-th largest

Quick Overview

Problem A — Validate parentheses

Problem B — k-th largest element using a heap

Submit Your Answer to Earn 20XP

Validate parentheses and find k-th largest

Quick Overview

Problem A — Validate parentheses

Problem B — k-th largest element using a heap

Submit Your Answer to Earn 20XP