The O(n) Club: Validate Stack Sequences: Because Stack Cheating Isn’t a Sport

The O(n) Club: Validate Stack Sequences — Because Stack Cheating Isn’t a Sport

⚡ TL;DR

Given two lists—one is the push parade, the other is the pop playlist—can they happen on a real LIFO stack? Don’t overthink it: actually simulate the process. Here’s the Java you wish you’d written before that last round of espresso:

public boolean validateStackSequences(int[] pushed, int[] popped) {
    Stack
<integer> stack = new Stack<>();
    int j = 0;
    for (int x : pushed) {
        stack.push(x);
        while (!stack.isEmpty() && stack.peek() == popped[j]) {
            stack.pop();
            j++;
        }
    }
    return stack.isEmpty();
}</integer>

🧠 How Devs Usually Mess This Up

Look, we’ve all tried to beat the system with pointer shenanigans or wacky inversions. Most common fails:

• No real stack simulation: Guessing without actually stacking anything is a ticket to Broken Logic Land.
• Popping ghosts: Trying to pop before the value’s even been pushed. Time travelers need not apply.
• Array stunt-doubling for stack: Using the array as a fake stack is error-prone if you sneeze on your indices.
• Ignoring boundaries: Loop off the end? Enjoy an exception and a headache.

🔍 What’s Actually Going On

Imagine a cafeteria. Trays stack up as each customer adds one (pushed). Hungry customers grab trays in a very specific order (popped). Can you really grab trays as requested, only taking what’s on top? Simulate: push in order, pop when the next pop value matches the top. If your stack’s empty at the end, it’s not haunted.

Real life: Undo/redo in your editor, bracket matching in a parser, even call stacks in your favorite debugger. If you don’t respect stack order, someone’s losing lunch… or their code.

🛠️ PseudoCode

• Start fresh: Create a real Stack and set a pointer j for popped.
• Push items: For every element in pushed, push onto the stack.
• Pop if you can: If the top of the stack is the current popped[j], actually pop and bump j up. Repeat as long as this holds.
• Wrap up: If the stack is empty when you’re done, the sequence is valid—nobody cheated.

💻 The Code

public boolean validateStackSequences(int[] pushed, int[] popped) {
    Stack
<integer> stack = new Stack<>();
    int j = 0;
    for (int x : pushed) {
        stack.push(x);
        while (!stack.isEmpty() && stack.peek() == popped[j]) {
            stack.pop();
            j++;
        }
    }
    return stack.isEmpty();
}</integer>

⚠️ Pitfalls, Traps & Runtime Smacks

• Popping before pushing: Don’t. Just… don’t.
• Index out-of-bounds joyride: Always check stack and indices before peeking or popping. Java isn’t shy about throwing exceptions. (Looking at you, ArrayIndexOutOfBoundsException.)
• Superfluous cleverness: Array-as-stack tricks or bitwise shenanigans save bytes, not brain cells. Interviewers want clarity.
• Complexity check: Both time and space are O(n). It’s not getting any snappier than that without burning the laws of physics.

🧠 Interviewer Brain-Tattoo

“If your Undo button ever lies about the past, update your stack logic—before you update your résumé.”