遞歸調用后的代碼是否僅在達到基本情況時才執行？

Question

我寫了一個遞歸來反轉一個鏈表：

def reverseList(self, head: ListNode) -> ListNode:

    #Base case
    if not head or not head.next:
        return head
    
    
    #Recurence relation
    second= head.next
    reverse = self.reverseList(second)
    second.next = head
    head.next = None
    
    return reverse

關於代碼的執行，我有一個簡單的問題，特別是對於遞歸關系。

我讀它的方式，遞歸關系的前兩行確保我們移動到並處理鏈表中的下一個節點。

但是，我的問題是，什么時候

second.next = head
head.next = None

被執行？ 是否僅在達到基本情況時？ 同時我們將每個部分完成的調用存儲在遞歸堆棧中？ 這是正確的嗎？

Answer 1

如果您好奇代碼是如何執行的，只需插入一些打印語句。

def countDown(amount, depth = 0):
    if amount == 0:
        print("finished")
        return amount
    amount -= 1
    print("entering depth", depth + 1, "amount is", amount)
    amount = countDown(amount, depth + 1)
    print("exiting depth", depth, "amount is", amount)
    return amount

countDown(10)

output 精美地表明 function 調用被放入堆棧並在提前返回發生時彈出

entering depth 1 amount is 9
entering depth 2 amount is 8
entering depth 3 amount is 7
entering depth 4 amount is 6
entering depth 5 amount is 5
entering depth 6 amount is 4
entering depth 7 amount is 3
entering depth 8 amount is 2
entering depth 9 amount is 1
entering depth 10 amount is 0
finished
exiting depth 9 amount is 0
exiting depth 8 amount is 0
exiting depth 7 amount is 0
exiting depth 6 amount is 0
exiting depth 5 amount is 0
exiting depth 4 amount is 0
exiting depth 3 amount is 0
exiting depth 2 amount is 0
exiting depth 1 amount is 0
exiting depth 0 amount is 0