I am trying to write a program to find leaf value sequences in python (https://leetcode.com/problems/leaf-similar-trees/description/?envType=study-plan-v2&envId=leetcode-75)

Here is my code

def leaves(root,ans) :
    if(root.left == None and root.right == None) :
        ans.append(root.val)
        return(ans) 
    elif (root.left == None ):
        ans = ans + leaves(root.right,ans)
        return(ans)  
    elif (root.right == None) :
        ans = ans + leaves(root.left,ans)
        return(ans)  
    else :
        ans = ans + leaves(root.left,ans) + leaves(root.right,ans)
        return(ans)    

For the case [1,2,3] in the 2nd example the function leaves([1,2,3],[]) should return [2,3] but instead returns [2,2,2,3]. I assume that the error is because the variable ans is a global variable. My understanding is that it should be a local variable each time when passed to a function. Please explain if my understanding is correct

Also what corrections do I need to make to the code