Dinner Plate Stacks
<p>You have an infinite number of stacks arranged in a row and numbered (left to right) from <code>0</code>, each of the stacks has the same maximum capacity.</p>

<p>Implement the <code>DinnerPlates</code> class:</p>

<ul>
	<li><code>DinnerPlates(int capacity)</code> Initializes the object with the maximum capacity of the stacks <code>capacity</code>.</li>
	<li><code>void push(int val)</code> Pushes the given integer <code>val</code> into the leftmost stack with a size less than <code>capacity</code>.</li>
	<li><code>int pop()</code> Returns the value at the top of the rightmost non-empty stack and removes it from that stack, and returns <code>-1</code> if all the stacks are empty.</li>
	<li><code>int popAtStack(int index)</code> Returns the value at the top of the stack with the given index <code>index</code> and removes it from that stack or returns <code>-1</code> if the stack with that given index is empty.</li>
</ul>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input</strong>
[&quot;DinnerPlates&quot;, &quot;push&quot;, &quot;push&quot;, &quot;push&quot;, &quot;push&quot;, &quot;push&quot;, &quot;popAtStack&quot;, &quot;push&quot;, &quot;push&quot;, &quot;popAtStack&quot;, &quot;popAtStack&quot;, &quot;pop&quot;, &quot;pop&quot;, &quot;pop&quot;, &quot;pop&quot;, &quot;pop&quot;]
[[2], [1], [2], [3], [4], [5], [0], [20], [21], [0], [2], [], [], [], [], []]
<strong>Output</strong>
[null, null, null, null, null, null, 2, null, null, 20, 21, 5, 4, 3, 1, -1]

<strong>Explanation:</strong> 
DinnerPlates D = DinnerPlates(2);  // Initialize with capacity = 2
D.push(1);
D.push(2);
D.push(3);
D.push(4);
D.push(5);         // The stacks are now:  2  4
                                           1  3  5
                                           ﹈ ﹈ ﹈
D.popAtStack(0);   // Returns 2.  The stacks are now:     4
                                                       1  3  5
                                                       ﹈ ﹈ ﹈
D.push(20);        // The stacks are now: 20  4
                                           1  3  5
                                           ﹈ ﹈ ﹈
D.push(21);        // The stacks are now: 20  4 21
                                           1  3  5
                                           ﹈ ﹈ ﹈
D.popAtStack(0);   // Returns 20.  The stacks are now:     4 21
                                                        1  3  5
                                                        ﹈ ﹈ ﹈
D.popAtStack(2);   // Returns 21.  The stacks are now:     4
                                                        1  3  5
                                                        ﹈ ﹈ ﹈ 
D.pop()            // Returns 5.  The stacks are now:      4
                                                        1  3 
                                                        ﹈ ﹈  
D.pop()            // Returns 4.  The stacks are now:   1  3 
                                                        ﹈ ﹈   
D.pop()            // Returns 3.  The stacks are now:   1 
                                                        ﹈   
D.pop()            // Returns 1.  There are no stacks.
D.pop()            // Returns -1.  There are still no stacks.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= capacity &lt;= 2 * 10<sup>4</sup></code></li>
	<li><code>1 &lt;= val &lt;= 2 * 10<sup>4</sup></code></li>
	<li><code>0 &lt;= index &lt;= 10<sup>5</sup></code></li>
	<li>At most <code>2 * 10<sup>5</sup></code> calls will be made to <code>push</code>, <code>pop</code>, and <code>popAtStack</code>.</li>
</ul>
Stack; Design; Hash Table; Heap (Priority Queue)
type DinnerPlates struct {
	capacity int
	stacks   [][]int
	notFull  *redblacktree.Tree
}

func Constructor(capacity int) DinnerPlates {
	return DinnerPlates{capacity: capacity, notFull: redblacktree.NewWithIntComparator()}
}

func (this *DinnerPlates) Push(val int) {
	if this.notFull.Size() == 0 {
		this.stacks = append(this.stacks, []int{val})
		if this.capacity > 1 {
			this.notFull.Put(len(this.stacks)-1, nil)
		}
	} else {
		index, _ := this.notFull.Left().Key.(int)
		this.stacks[index] = append(this.stacks[index], val)
		if len(this.stacks[index]) == this.capacity {
			this.notFull.Remove(index)
		}
	}
}

func (this *DinnerPlates) Pop() int {
	return this.PopAtStack(len(this.stacks) - 1)
}

func (this *DinnerPlates) PopAtStack(index int) int {
	if index < 0 || index >= len(this.stacks) || len(this.stacks[index]) == 0 {
		return -1
	}
	val := this.stacks[index][len(this.stacks[index])-1]
	this.stacks[index] = this.stacks[index][:len(this.stacks[index])-1]
	if index == len(this.stacks)-1 && len(this.stacks[index]) == 0 {
		for len(this.stacks) > 0 && len(this.stacks[len(this.stacks)-1]) == 0 {
			this.notFull.Remove(len(this.stacks) - 1)
			this.stacks = this.stacks[:len(this.stacks)-1]
		}
	} else {
		this.notFull.Put(index, nil)
	}
	return val
}

/**
 * Your DinnerPlates object will be instantiated and called as such:
 * obj := Constructor(capacity);
 * obj.Push(val);
 * param_2 := obj.Pop();
 * param_3 := obj.PopAtStack(index);
 */