Leetcode Minstack JavaScript Solution

Introduction

Originally posted here With this post, I would like to share a low level Javascript solution for one of the most popular design problems from leetcode it is called MinStack.The problem expect a stack that supports push, pop, top, and retrieving the minimum element in constant time. Here instead of using Javascript's length, Math.min(), push, pop etc. , the expected functions are written at the low level as possible.

Final Code

/**
 * initialize data structure
 */
var MinStack = function () {
  this.stack = []
  this.minVals = []
  this.count = 0
  this.min = Number.MAX_VALUE
}

/**
 * @param {number} x
 * @return {void}
 */
MinStack.prototype.push = function (x) {
  if (x < this.min || this.count === 0) {
    this.min = x
  }

  this.minVals[this.count] = this.min
  this.stack[this.count] = x
  this.count++
}

/**
 * @return {void}
 */
MinStack.prototype.pop = function () {
  delete this.stack[this.count - 1]
  delete this.minVals[this.count - 1]
  this.min = this.minVals[this.count - 2]
  this.count--
}

/**
 * @return {number}
 */
MinStack.prototype.top = function () {
  return this.stack[this.count - 1]
}

/**
 * @return {number}
 */
MinStack.prototype.getMin = function () {
  if (this.count > 0) return this.minVals[this.count - 1]
  else if (this.count === 0) return this.MinVals[0]
}

MinStack

• stack: is a storage of the values, in other words, the stack
• minVals: is a storage for the minimum Values; it is needed because after pop() operation we might delete the minimum value of the stack so the minimum values are also should be tracked.
• count: instead of using Javascript's length we can use this.count in order to set the last index and get the value of stack's last value.
• min: this is a kind of initial and global minimum value, that starts with the maximum numeric value in Javascript.

push(x)

Adds an element to the stack. Does not return any value. Here, while adding new element(x) to the stack we also check that whether new element is less than the this.min value. Also include the case that, if the stack is empty, means this.count===0, then of course the first element of this.minVals is also equal to the new element(x). Both length of the stacks are equal which is the value of count. Then instead of using Javascript's build in push() function, we say the last element of the stack is the new element:

...

    if (x < this.min || this.count === 0) {
        this.min = x;
    }

    this.minVals[this.count] = this.min;
    this.stack[this.count] = x;
    this.count++;

pop()

Removes the last item from the stack. We are not going to use pop() function of Javascript. We just going to delete the last item from the stack. We need to consider that, maybe the value which is going to be removed is the minimum value of the array. So actually thats why we've needed an extra minVals stack instead of only with a this.min. In order to catch the new state of the stack, we need to also delete the last item of the minVals stack. But! We also should remember that when we add new value to the stack with our push(x) function that is given above. There, we compare the x value with the this.min value, thats why this.min value is no longer the last item of the this.minVals stack but the previous one. And lastly, since we removed the last item from stack we should decrease the count number, so that when next time we do another operation, it should follow the last index of the stack with the count value.

...
    delete this.stack[this.count - 1];
    this.min = this.minVals[this.count - 2];
    delete this.minVals[this.count - 1];
    this.count--;

top()

Returns the last item of the stack. Here the length of the stack is equal to this.count and the last item of the stack is at the index of this.count-1:

return this.stack[this.count - 1]

getMin()

Returns the minium value of the stack. Here instead of scanning the whole stack, since we already set the minimum value to the minStack all the time. We can comfartably return the last item of minStack. Remember the last item's index is this.count-1; but if we are already at count=0 then we simply should return the first value of the minStack:

...
if (this.count === 0)
        return this.MinVals[0]
    else
        return this.minVals[this.count - 1];