4-algos-stack.ts

// BE SURE TO IMPORT YOUR STACK CLASS

// ==============================
// 1️⃣ Reverse a String Using a Stack
// ==============================
// Write a function that takes a string as input and returns the reversed string using a stack.
// You may only use stack operations (`push`, `pop`, `isEmpty`).

// Example Test Cases:
// reverseString("hello") // "olleh"
// reverseString("world") // "dlrow"
// reverseString("") // ""
// reverseString("abcd") // "dcba"

// ==============================
// 2️⃣ Check for Balanced Parentheses
// ==============================
// Given a string containing only the characters `()`, `{}`, and `[]`,
// write a function to determine if the string is valid.
// A string is valid if brackets are closed in the correct order. Use a stack to track open brackets.

// Example Test Cases:
// isValidParentheses("({[]})") // true
// isValidParentheses("({[)]}") // false
// isValidParentheses("()") // true
// isValidParentheses("{[()]}") // true
// isValidParentheses("(((") // false

// ==============================
// 3️⃣ Evaluate a Postfix Expression
// ==============================
// Write a function that evaluates a mathematical expression in **postfix notation** (Reverse Polish Notation).
// The function should use a stack to process numbers and operators.
// Assume the input is a space-separated string of numbers and `+`, `-`, `*`, or `/` operators.

// Example Test Cases:
// evaluatePostfix("3 4 +") // 7
// evaluatePostfix("5 1 2 + 4 * + 3 -") // 14
// evaluatePostfix("10 2 8 * + 3 -") // 23
// evaluatePostfix("6 2 /") // 3
// evaluatePostfix("4 5 * 2 /") // 10

// ==============================
// 4️⃣ Next Greater Element
// ==============================
// Given an array of integers, find the **next greater element** for each element.
// The next greater element of an element **x** is the first element to the right that is greater than **x**.
// If none exists, return `-1` for that element. Use a stack for efficiency.

// Example Test Cases:
// nextGreaterElement([4, 5, 2, 10, 8]) // [5, 10, 10, -1, -1]
// nextGreaterElement([3, 2, 1]) // [-1, -1, -1]
// nextGreaterElement([1, 3, 2, 4]) // [3, 4, 4, -1]

// ==============================
// 5️⃣ Daily Temperatures
// ==============================
// Given an array `temperatures` where `temperatures[i]` is the temperature on day `i`,
// return an array **answer** where `answer[i]` is the number of days you have to wait after the `i-th` day
// to get a warmer temperature. If there is no future day with a warmer temperature, return `0`.

// Example Test Cases:
// dailyTemperatures([73, 74, 75, 71, 69, 72, 76, 73]) // [1, 1, 4, 2, 1, 1, 0, 0]
// dailyTemperatures([30, 40, 50, 60]) // [1, 1, 1, 0]
// dailyTemperatures([30, 20, 10]) // [0, 0, 0]