def reverse(self, x):
    """
    :type x: int
    :rtype: int
    """
    s = list(str(x))
    if s[0] == "-":
        #if s=['-','1','2','3'], then s[1:]=['1','2','3'], after running self.reverseCore, s still is ['-','1','2','3']
        print(id(s[1:]))
        self.reverseCore(s[1:])
        print(s)
    else:
        #if s=['1','2','3'], after running self.reverseCore, s will be['3','2','1']
        self.reverseCore(s)
        print(s)
def reverseCore(self, s):
    print(id(s))
    if len(s)<= 1:
        return
    l = len(s)
    k = l//2
    for i in range(0, k):
        s[i], s[l-i-1] = s[l-i-1], s[i]