Photo by Glenn Carstens-Peters on Unsplash

# Mastering Stack Interview Questions: A Comprehensive Guide

Some main concepts:

**Nearest Greater Left**:Maintain a stack to store elements encountered so far.

Iterate through the array from left to right.

For each element, pop elements from the stack until you find an element greater than the current element or the stack becomes empty. The top element of the stack will be the nearest greater element to the left.

If the stack becomes empty, there's no greater element to the left.

**Nearest Greater Right**:Similar to the Nearest Greater Left, but iterate through the array from right to left.

Pop elements from the stack until you find a greater element or the stack becomes empty. The top element of the stack will be the nearest greater element to the right.

**Nearest Smaller Left**:- Similar to Nearest Greater Left, but instead of finding greater elements, find smaller elements.

**Nearest Smaller Right**:- Similar to Nearest Smaller Left, but iterate through the array from right to left.

**Stock Span Problem**:Maintain a stack to store pairs of (index, span).

Iterate through the array of stock prices.

For each price, pop elements from the stack until you find a price greater than the current price or the stack becomes empty.

Calculate the span by subtracting the index of the popped element from the current index.

Push the current index and span onto the stack.

**Maximum Area of Histogram**:Use a stack to maintain indices of histogram bars in non-decreasing order of heights.

Iterate through the histogram bars.

For each bar, while the stack is not empty and the current bar's height is less than the height of the bar at the top of the stack, calculate the area of the rectangle with the top of the stack as the minimum height.

Update the maximum area accordingly.

Push the current bar's index onto the stack.

**Maximum Area of Rectangle in Binary Maze**:- You can use the Maximum Area of Histogram algorithm to solve this problem efficiently. Convert each row of the binary matrix into a histogram, and then find the maximum area of a rectangle using the histogram.

**Rain Water Trapping**:- This problem can also be solved using two stacks to keep track of the left and right boundaries of potential water traps.

**Implement a Uni stock**:- Implementing a stock system typically doesn't require stack data structure unless you're keeping track of transactions. However, stack operations might be involved in maintaining the transaction history.

**Implement Stack using Heap**:- You can implement a stack using a max heap, where the key of each element is its position in the stack.

**The Celebrity Problem**:- You can use a stack-based approach to eliminate potential candidates for being a celebrity based on who knows whom.

**Longest Valid Parenthesis**:- You can use a stack to keep track of the indices of '(' characters encountered so far. When encountering a ')', pop the top index from the stack and calculate the length of the valid parenthesis substring.

**Iterative Tower of Hanoi**:- The Tower of Hanoi problem is traditionally solved recursively, but it can also be solved iteratively using a stack to simulate the recursive calls.