stack

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Stack Algorithm Patterns

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Concept

The Stack pattern is based on the Last-In-First-Out (LIFO) principle and is used to process elements in a controlled order, often when the most recent element affects the next decision. Stack-based problems typically involve matching, reversing, validating, or maintaining a monotonic order, making stacks ideal for parsing expressions, handling nested structures, and solving “previous/next greater” type problems.

When to Use

• When the problem involves nested structures (parentheses, brackets, scopes).
• When you need to undo or rollback previous operations.
• When tracking previous or next greater/smaller elements.
• When order matters and recent elements influence current logic.

Common Operations

• push and pop elements based on conditions.
• Using a monotonic stack (increasing or decreasing).
• Storing indices instead of values to compute distances or ranges.
• Simulating recursion or state using an explicit stack.

Examples

#	Name	Java	Python	Go	Link	Company
1	Next Greater Element I	✅	❌	❌	LeetCode #496
2	Valid Parentheses	✅	❌	❌	LeetCode #20
3	Implement Queue using Stacks	✅	❌	❌	LeetCode #232	#tagA

⬅️ Sliding Window 🔸 Tree Traversal ➡️

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