aloalgoStack Applications and Patterns
Stacks are useful whenever you need to track "the most recent" item or reverse the order of operations. Here are the most common patterns you'll encounter in interviews.Pattern 1: Matching Parentheses
The classic stack problem: check if brackets are properly matched and nested. Push opening brackets, pop when you see closing brackets.Pattern 2: Evaluate Expressions
Stacks are essential for evaluating mathematical expressions. Reverse Polish Notation (postfix) is especially clean with a stack.Pattern 3: Reversing
Stacks naturally reverse order since LIFO means first-in is last-out.Pattern 4: Next Greater Element
Find the next element that is greater than the current one. This uses a monotonic decreasing stack.Pattern 5: DFS Using Stack
Depth-first search can be implemented iteratively using an explicit stack instead of recursion. This avoids stack overflow for deep graphs.Pattern 6: Min Stack
Design a stack that supports push, pop, and getMin all in O(1). The trick is to maintain a second stack that tracks the minimum at each level.Pattern 7: Decode String
Use a stack to handle nested patterns like "3[a2[c]]" → "accaccacc".Pattern 8: Daily Temperatures
Given daily temperatures, find how many days until a warmer day. This is a variation of "next greater element".Common Interview Tips
- Always check if stack is empty: Before calling pop() or accessing the top, verify the stack isn't empty.
- Think about what to store: Sometimes you store values, sometimes indices, sometimes tuples of (value, extra_info).
- Monotonic stacks: For "next greater/smaller" problems, maintain a stack in sorted order.
- Nested structures: When you see brackets or nested patterns, think stack immediately.
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