Why does some solution work on VSCode but not LeetCode?

Question

I tried to resolve this Leetcode problem with the solution as follows on Visual Studio Code :

class CustomStack:
    vals = list()

    def __init__(self, maxSize):
        self.maxSize = maxSize

    def push(self, x):
        if len(self.vals) < self.maxSize:
            self.vals.append(x)
        print(self.vals)

    def pop(self):
        if len(self.vals) > 0:
            val = self.vals.pop()
            print(val)
            return val
        else:
            print(-1)
            return -1

    def increment(self, k, val):
        k = min(k, len(self.vals))
        for i in range(k):
            self.vals[i] += val
        print(self.vals)

It prints correct numbers on VSCode: [null,null,34,null,-1,null,null,63,null,null,null,null]

But it fails on LeetCode which prints [null,null,85,null,181,null,null,196,null,null,null,null]

I know the correct solution can be this:

class CustomStack:
    def __init__(self, maxSize):
        self.vals =[]
        self.maxSize = maxSize

But why the original version works on VSCode, not Leetcode?

`vals` in your code is a class variable, not on the instance. Leetcode is probably running multiple tests using your code. Each test will share `vals`. Look at this question: https://stackoverflow.com/questions/1680528/how-to-avoid-having-class-data-shared-among-instances — dspencer, Mar 15 '20 at 05:10

score 2 · Accepted Answer · answered Mar 15 '20 at 05:16

When running your program in VS Code, you are testing a single example using one instance of the CustomStack class. In LeetCode, multiple test cases are executed using a new instance of the CustomStack class. Since vars is a class variable, not an instance variable, it is being shared across all of the CustomStack instances created during execution of your program. This means that each instance modifies the same list.

You have already shown that the correct way to implement a variable specific to an instance of a class is by defining it during the __init__ method:

class CustomStack:
    def __init__(self, maxSize):
        self.vals =[]
        self.maxSize = maxSize

Why does some solution work on VSCode but not LeetCode?

1 Answers1