有如下Python程序：

def fun(s):
    st = s[1]
    for i in range(1, len(s)):
        if len(st) == 0 or s[i] != st[len(st)-1]:
            st = st + s[i]
        else:
            st = st[0:len(st)-1]
    print(st)
fun("xxyyzzxyz")