Problem

Design a stack that supports push, pop, and min operations in constant time.
Example input:
push(5), push(3), push(7), push(2), min()
Example output:
2

Solution

// Solution:
// The solution is to use a second stack to keep track of the minimum value.
// When a new value is pushed onto the stack, it is also pushed onto the min stack if it is less than or equal to the current min value.
// When a value is popped off the stack, it is also popped off the min stack if it is equal to the current min value.
// This way, the min stack will always have the current min value at the top.
// This solution is optimal because it allows for constant time min operations.

class Stack {
  constructor() {
    this.stack = [];
    this.minStack = [];
  }

  push(value) {
    this.stack.push(value);
    if (this.minStack.length === 0 || value <= this.minStack[this.minStack.length - 1]) {
      this.minStack.push(value);
    }
  }

  pop() {
    const value = this.stack.pop();
    if (value === this.minStack[this.minStack.length - 1]) {
      this.minStack. pop();
    }
    return value;
  }

  min() {
    return this.minStack[this.minStack.length - 1];
  }
}

const stack = new Stack();
stack.push(5);
stack.push(3);
stack.push(7);
stack.push(2);
console.log(stack.min()); // 2
stack.pop();
console.log(stack.min()); // 3
stack.pop();
console.log(stack.min()); // 3
stack.pop();
console.log(stack.min()); // 5
stack.pop();
console.log(stack.min()); // undefined