Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining. This is a famous interview problem to learn time and space complexity optimization using various approaches. Two-pointers approach provides an efficient solution in O(n) time and O(1) space.

Given a stack, write a program to sort the stack in ascending order. We are not allowed to make any assumptions about how the stack is implemented. The only functions to be used are push(s, x), pop(s), top(s), isEmpty(s).

Write a program to implement a stack using the queues. The implemented stack should support standard operations like push(x), pop(), and top(). This is an excellent problem to learn problem-solving and visualize the use case of stack and queue operations.

Write a program to Implement a queue using the stack. The implemented queue should support standard operations like enqueue, dequeue, and front. This is an excellent problem to learn problem-solving and visualize the use case of stack and queue operations.

Given an array, find the next greater element for every element in the array. The next greater element for an element is the first greater element on the right side of the array. This is one of the best problems to learn problem-solving using stack.

In recursive DFS traversals of a binary tree, we have three basic elements to traverse— root, left subtree, and right subtree. The traversal order depends on the order in which we process the root node. Here recursive code is simple and easy to visualize — only one function parameter and 3–4 lines of code. So critical question would be — How can we convert it into iterative code using stack? To simulate the recursive traversal into an iterative traversal, we need to understand the flow of recursive calls.