Not sure why this happened
ansonchang
38

I was doing the question Validate Stack Sequences. My first attempt was this

class Solution:
    def validateStackSequences(self, pushed: List[int], popped: List[int], stack = []) -> bool:
        print(pushed, popped, stack)
        if popped == []:
            return True
        else:
            if popped[0] not in stack:
                stack.append(pushed.pop(0))
                return self.validateStackSequences(pushed, popped, stack)
            elif popped[0] == stack[-1]:
                stack.pop()
                popped.pop(0)
                return self.validateStackSequences(pushed, popped, stack)
            else:
                return False

When it ran against test case pushed = [0,1] popped = [1,0] I printed out the results and my stack = [1,2]
However, when I implemented a helper function

class Solution:
    def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
        return self.helper(pushed, popped, stack = [])
    
    def helper(self, pushed, popped, stack):
        if popped == []:
            return True
        else:
            if popped[0] not in stack:
                stack.append(pushed.pop(0))
                return self.helper(pushed, popped, stack)
            elif popped[0] == stack[-1]:
                stack.pop()
                popped.pop(0)
                return self.helper(pushed, popped, stack)
            else:
                return False

It passed all test cases.
Is this something due to how leetcode runs the test cases?

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