Finding longest substring in alphabetical order

Question

EDIT: I am aware that a question with similar task was already asked in SO but I\'m interested to find out the problem in this specific piece of code. I am

Answer 1

s = 'gkuencgybsbezzilbfg'
x = s.lower()
y = ''
z = [] #creating an empty listing which will get filled

for i in range(0,len(x)):
    if i == len(x)-1:
       y = y + str(x[i])
       z.append(y)
       break 
    a = x[i] <= x[i+1]
    if a == True:
        y = y + str(x[i])
    else: 
        y = y + str(x[i])
        z.append(y)  # fill the list 
        y = ''
# search of 1st longest string        
L = len(max(z,key=len))      # key=len takes length in consideration 
for i in range(0,len(z)):
    a = len(z[i])
    if a == L:   
        print 'Longest substring in alphabetical order is:' + str(z[i])
        break