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常见错误

1. No Base Case​

def factorial(n):          # ❌ Missing base case
    return n * factorial(n - 1)

Causes infinite recursion → stack overflow.

Fix: Always add a base case check (if n <= 1: return 1).

2. No Return Statement​

def factorial(n):
    if n <= 1:
        return 1
    else:
        factorial(n - 1)   # ❌ Missing return

The recursive result is computed but not returned → function returns None.

Fix: return n * factorial(n - 1)

3. Problem Doesn't Shrink​

def countdown(n):
    if n == 0:
        return
    else:
        return countdown(n)   # ❌ n never decreases

Same n passed every time → infinite recursion.

Fix: Pass n - 1 (or a smaller value).

4. Wrong Base Case Condition​

def factorial(n):
    if n == 1:      # ❌ Fails when n = 0
        return 1
    else:
        return n * factorial(n - 1)

Calling factorial(0) causes infinite recursion because n skips past 1.

Fix: Use n &lt;= 1 instead of n == 1.

5. Modifying the Parameter​

def factorial(n):
    if n <= 1:
        return 1
    else:
        n = n - 1              # ❌ Don't modify the parameter
        return n * factorial(n)

Modifying n before the recursive call changes the multiplication value.

Fix: Pass n - 1 directly: return n * factorial(n - 1)

6. Confusing Iteration and Recursion​

def sum_array(arr):
    total = 0
    if len(arr) == 0:          # ❌ Mixed loop + recursion
        return 0
    for i in range(len(arr)):
        total += arr[i]
    return total + sum_array(arr)  # ❌ No shrinking argument

Fix: Write pure recursion with index parameter.

7. Stack Overflow for Deep Recursion​

Very deep recursion (e.g. n = 10000) may overflow the call stack.

Hint for exams: If the question says "recursive function for a large n", consider the depth limit.

Quick Checklist​

CheckOK?
Base case exists?☐
Base case returns a value?☐
Recursive call argument is smaller?☐
Recursive result is returned?☐
All branches have return?☐
Function works for edge cases?☐