pq refactoring

follow container/heap example from std doc.

priority_queue.go

@@ -20,17 +20,18 @@ func (pq priorityQueue) Swap(i, j int) {
 }
 
 func (pq *priorityQueue) Push(x interface{}) {
-	s := *pq
+	n := len(*pq)
 	bar := x.(*Bar)
-	bar.index = len(s)
-	s = append(s, bar)
-	*pq = s
+	bar.index = n
+	*pq = append(*pq, bar)
 }
 
 func (pq *priorityQueue) Pop() interface{} {
-	s := *pq
-	*pq = s[0 : len(s)-1]
-	bar := s[len(s)-1]
+	old := *pq
+	n := len(old)
+	bar := old[n-1]
+	old[n-1] = nil // avoid memory leak
 	bar.index = -1 // for safety
+	*pq = old[:n-1]
 	return bar
 }