func (p *PriorityQueue) Pop() interface{} {
n := p.Len()
p.Swap(0, n-1)
heap.Down(p, 0, n-1)
val := p.List[n-1]
p.List = p.List[0 : n-1]
return val
}
func heapSort(a []int) {
arr := Array(a)
heap.Init(arr)
for i := len(arr) - 1; i > 0; i-- {
arr.Swap(0, i)
heap.Down(arr, 0, i)
}
}