Dinner Plate Stacks LeetCode solution in JavaScript

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/**
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 * @param {number} capacity
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 */
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var DinnerPlates = function (capacity) {
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  this.capacity = capacity;
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  this.stacks = [];
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};
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/**
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 * @param {number} val
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 * @return {void}
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 */
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DinnerPlates.prototype.push = function (val) {
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  var needNewStack = true;
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  for (var i = 0; i < this.stacks.length; i++) {
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    if (this.stacks[i].length < this.capacity) {
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      this.stacks[i].push(val);
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      needNewStack = false;
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      break;
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    }
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  }
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  if (needNewStack) {
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    this.stacks.push([val]);
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  }
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};
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/**
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 * @return {number}
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 */
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DinnerPlates.prototype.pop = function () {
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  var val = -1;
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  for (var i = this.stacks.length - 1; i >= 0; i--) {
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    if (this.stacks[i].length > 0) {
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      val = this.stacks[i].pop();
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      break;
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    }
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  }
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  return val;
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};
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/**
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 * @param {number} index
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 * @return {number}
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 */
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DinnerPlates.prototype.popAtStack = function (index) {
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  // console.log(index, this.stacks, ...this.stacks[index])
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  var val = -1;
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  if (this.stacks[index] && this.stacks[index].length > 0) {
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    val = this.stacks[index].pop();
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  }
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  return val;
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};
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/**
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 * Your DinnerPlates object will be instantiated and called as such:
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 * var obj = new DinnerPlates(capacity)
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 * obj.push(val)
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 * var param_2 = obj.pop()
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 * var param_3 = obj.popAtStack(index)
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 */