Below is a high-yield, pattern-driven set of stack problems. This covers all core concepts:

• LIFO behavior
• monotonic stacks
• parsing / evaluation
• recursion simulation
• next greater/smaller patterns

1. Basic Stack Usage (Validation)

Core Skill

• Push / pop matching elements

Problems

• Valid Parentheses
• Min Stack

Concepts

• Matching pairs
• Maintaining auxiliary state (min stack)

2. Monotonic Stack (CRITICAL)

Core Skill

• Maintain increasing or decreasing order