155 Min Stack

Problem Statement

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

Implement the MinStack class:

MinStack() initializes the stack object.
void push(int val) pushes the element val onto the stack.
void pop() removes the element on the top of the stack.
int top() gets the top element of the stack.
int getMin() retrieves the minimum element in the stack.

You must implement a solution with O(1) time complexity for each function.

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[|],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

Approach

We use two stacks, one as the "result" stack and one to keep track of minimum Values.
Push: If the minStack is empty and the current value is smaller/equal than the last element on the minStack we push the value onto it.
Pop: We pop from the stack and if that value equals the last value on the minStack we also pop from the minStack.

The other operations are straightforward

Solution

var MinStack = function() {
  this.stack = []
  this.minStack = []
};

MinStack.prototype.push = function(val) {
  this.stack.push(val)
  if (!this.minStack.length || val <= this.minStack[this.minStack.length - 1]) {
    this.minStack.push(val)
  }
};

MinStack.prototype.pop = function() {
  let x = this.stack.pop()
  if (x === this.minStack[this.minStack.length - 1]) {
    this.minStack.pop()
  }
};

MinStack.prototype.top = function() {
  return this.stack[this.stack.length - 1]
};

MinStack.prototype.getMin = function() {
  return this.minStack[this.minStack.length - 1]
};

Questions

References