||Topic
Stack Problems
Stack problems — parentheses matching, monotonic stacks, expression evaluation, and next-greater-element.
Easy · 1
Stacks are deceptively simple — push, pop, peek — but they unlock an entire family of problems where order matters and you need to reach back to the most-recent unfinished work. The two flavors that show up in interviews are bracket matching (validate, balance, or simulate parentheses) and monotonic stacks (find the next greater or next smaller element in O(n)).
The monotonic-stack pattern is worth learning cold. It solves a class of problems — daily temperatures, largest rectangle in histogram, next greater element, sum of subarray minimums — that look hopelessly O(n²) at first glance but reduce to O(n) once you keep a stack of pending elements ordered by value. The trick is realizing that once an element is popped, it can never come back, so the total work across all iterations is amortized linear.
The Ratta stack track covers Valid Parentheses, Min Stack, Evaluate Reverse Polish Notation, Daily Temperatures, and Largest Rectangle in Histogram. Each problem is solved with the canonical pattern, complete with C++, Java, Python, and Go implementations and worked-out complexity analysis.