{ "id": "stacks-queues/stack", "version": 1, "deterministic": true, "url": "/dsa/stacks-queues/stack", "topic": { "slug": "stacks-queues", "title": "Stacks & Queues" }, "title": "Stack", "tagline": "Last in, first out. The undo button of data structures.", "vizKind": "stack", "complexity": { "time": { "best": "O(1) push", "average": "O(1) pop", "worst": "O(1) peek" }, "space": "O(n)", "growth": "constant", "note": "Push, pop and peek all take the same time whether the stack holds 3 items or 3 million. Nothing has to shuffle, because nothing below the top ever moves." }, "code": { "javascript": "class Stack {\n constructor() { this.items = []; }\n push(value) {\n this.items.push(value);\n }\n pop() {\n if (this.isEmpty()) return null;\n return this.items.pop();\n }\n peek() {\n return this.items[this.items.length - 1];\n }\n isEmpty() {\n return this.items.length === 0;\n }\n}", "python": "class Stack:\n def __init__(self): self.items = []\n\n def push(self, value):\n self.items.append(value)\n\n def pop(self):\n if self.is_empty(): return None\n return self.items.pop()\n\n def peek(self):\n return self.items[-1]\n\n def is_empty(self):\n return len(self.items) == 0\n", "java": "class Stack {\n private List items = new ArrayList<>();\n\n void push(int value) {\n items.add(value);\n }\n Integer pop() {\n if (isEmpty()) return null;\n return items.remove(items.size() - 1);\n }\n Integer peek() {\n return items.get(items.size() - 1);\n }\n boolean isEmpty() {\n return items.isEmpty();\n }\n}", "cpp": "class Stack {\n vector items;\npublic:\n void push(int value) {\n items.push_back(value);\n }\n int pop() {\n if (isEmpty()) return -1;\n int top = items.back();\n items.pop_back();\n return top;\n }\n int peek() { return items.back(); }\n bool isEmpty() {\n return items.empty();\n }\n};" }, "defaultInput": [ 3, 7, 1, 9 ], "defaultTarget": null, "inputHint": "These get pushed one by one. Then we peek, and pop twice.", "pitfalls": [ { "wrong": "Popping without checking whether the stack is empty.", "right": "Popping an empty stack is a classic crash. Always ask isEmpty() first, or make pop return null." }, { "wrong": "Confusing peek with pop.", "right": "peek looks at the top and leaves it there. pop takes it away. Using pop when you meant peek quietly destroys your data." }, { "wrong": "Trying to reach an item in the middle.", "right": "You can't — not without popping everything above it. If you need that, a stack is the wrong structure and you probably want an array." } ], "quiz": [ { "q": "You push 1, 2, 3, then pop once. What comes out?", "options": [ "1", "2", "3", "Nothing" ], "answer": 2, "why": "3 went on last, so 3 comes off first. Last in, first out." }, { "q": "Why is 'stack overflow' called that?", "options": [ "It's just a website name", "Function calls pile onto a stack, and infinite recursion fills it up", "The stack sorts itself and overflows", "It means you ran out of disk space" ], "answer": 1, "why": "Every function call is pushed onto the call stack. Recursion with no base case pushes forever, until the stack has no room left. It literally overflows." }, { "q": "How long does it take to push onto a stack holding a million items?", "options": [ "A million steps", "About 20 steps", "One step", "It depends on the values" ], "answer": 2, "why": "One. You only ever touch the top — the size of the pile beneath it is irrelevant. That's O(1)." } ], "problems": [ { "name": "Valid Parentheses", "difficulty": "Easy", "url": "https://leetcode.com/problems/valid-parentheses/" }, { "name": "Min Stack", "difficulty": "Medium", "url": "https://leetcode.com/problems/min-stack/" }, { "name": "Evaluate Reverse Polish Notation", "difficulty": "Medium", "url": "https://leetcode.com/problems/evaluate-reverse-polish-notation/" } ], "trace": { "note": "Each frame is one immutable step: data snapshot, pointers, per-index roles, executing code line, and a one-sentence explanation.", "length": 11, "frames": [ { "data": { "items": [] }, "pointers": [], "highlights": [], "codeLine": 1, "variables": { "size": 0 }, "explanation": "A stack is a pile of plates. You add to the top, and you take from the top. The bottom plate is trapped until everything above it is gone.", "stats": { "comparisons": 0, "swaps": 0 } }, { "data": { "items": [ { "id": "s0-3", "value": 3 } ] }, "pointers": [ { "name": "top", "index": 0 } ], "highlights": [ { "index": 0, "role": "swap" } ], "codeLine": 4, "variables": { "pushed": 3, "size": 1 }, "explanation": "push(3) — 3 goes on top of the pile.", "stats": { "comparisons": 0, "swaps": 0 } }, { "data": { "items": [ { "id": "s0-3", "value": 3 }, { "id": "s1-7", "value": 7 } ] }, "pointers": [ { "name": "top", "index": 1 } ], "highlights": [ { "index": 1, "role": "swap" } ], "codeLine": 4, "variables": { "pushed": 7, "size": 2 }, "explanation": "push(7) — 7 goes on top of the pile.", "stats": { "comparisons": 0, "swaps": 0 } }, { "data": { "items": [ { "id": "s0-3", "value": 3 }, { "id": "s1-7", "value": 7 }, { "id": "s2-1", "value": 1 } ] }, "pointers": [ { "name": "top", "index": 2 } ], "highlights": [ { "index": 2, "role": "swap" } ], "codeLine": 4, "variables": { "pushed": 1, "size": 3 }, "explanation": "push(1) — 1 goes on top of the pile.", "stats": { "comparisons": 0, "swaps": 0 } }, { "data": { "items": [ { "id": "s0-3", "value": 3 }, { "id": "s1-7", "value": 7 }, { "id": "s2-1", "value": 1 }, { "id": "s3-9", "value": 9 } ] }, "pointers": [ { "name": "top", "index": 3 } ], "highlights": [ { "index": 3, "role": "swap" } ], "codeLine": 4, "variables": { "pushed": 9, "size": 4 }, "explanation": "push(9) — 9 goes on top of the pile.", "stats": { "comparisons": 0, "swaps": 0 } }, { "data": { "items": [ { "id": "s0-3", "value": 3 }, { "id": "s1-7", "value": 7 }, { "id": "s2-1", "value": 1 }, { "id": "s3-9", "value": 9 } ] }, "pointers": [ { "name": "top", "index": 3 } ], "highlights": [ { "index": 3, "role": "found" } ], "codeLine": 11, "variables": { "peeked": 9, "size": 4 }, "explanation": "peek() — look at the top plate (9) without taking it. The pile is unchanged.", "stats": { "comparisons": 0, "swaps": 0 } }, { "data": { "items": [ { "id": "s0-3", "value": 3 }, { "id": "s1-7", "value": 7 }, { "id": "s2-1", "value": 1 }, { "id": "s3-9", "value": 9 } ] }, "pointers": [ { "name": "top", "index": 3 } ], "highlights": [ { "index": 3, "role": "compare" } ], "codeLine": 7, "variables": { "popping": 9, "size": 4 }, "explanation": "pop() — the top plate is 9. That is the one that comes off.", "stats": { "comparisons": 0, "swaps": 0 } }, { "data": { "items": [ { "id": "s0-3", "value": 3 }, { "id": "s1-7", "value": 7 }, { "id": "s2-1", "value": 1 } ] }, "pointers": [ { "name": "top", "index": 2 } ], "highlights": [ { "index": 2, "role": "active" } ], "codeLine": 8, "variables": { "popped": 9, "size": 3 }, "explanation": "9 is gone. 1 is the new top — the last one that went in is the first one out.", "stats": { "comparisons": 0, "swaps": 0 } }, { "data": { "items": [ { "id": "s0-3", "value": 3 }, { "id": "s1-7", "value": 7 }, { "id": "s2-1", "value": 1 } ] }, "pointers": [ { "name": "top", "index": 2 } ], "highlights": [ { "index": 2, "role": "compare" } ], "codeLine": 7, "variables": { "popping": 1, "size": 3 }, "explanation": "pop() — the top plate is 1. That is the one that comes off.", "stats": { "comparisons": 0, "swaps": 0 } }, { "data": { "items": [ { "id": "s0-3", "value": 3 }, { "id": "s1-7", "value": 7 } ] }, "pointers": [ { "name": "top", "index": 1 } ], "highlights": [ { "index": 1, "role": "active" } ], "codeLine": 8, "variables": { "popped": 1, "size": 2 }, "explanation": "1 is gone. 7 is the new top — the last one that went in is the first one out.", "stats": { "comparisons": 0, "swaps": 0 } }, { "data": { "items": [ { "id": "s0-3", "value": 3 }, { "id": "s1-7", "value": 7 } ] }, "pointers": [], "highlights": [], "codeLine": 14, "variables": { "size": 2, "isEmpty": false }, "explanation": "The stack holds 2 items. Every push and every pop touched only the top — which is why both are instant, no matter how tall the pile gets.", "stats": { "comparisons": 0, "swaps": 0 } } ] } }