Problem:
Problem Statement

We found a dictionary of the Ancient Civilization Mayo (ACM) during excavation of the ruins.
After analysis of the dictionary, we revealed they used a language that had not more than 26 letters.
So one of us mapped each letter to a different English alphabet and typed all the words in the dictionary into a computer.

How the words are ordered in the dictionary, especially whether they are ordered lexicographically, is an interesting topic to many people.
As a good programmer, you are requested to write a program to judge whether we can consider the words to be sorted in a lexicographical order.

Note:
In a lexicographical order, a word always precedes other words it is a prefix of.
For example, ab
precedes abc,
abde, and so on.

Input

The input consists of multiple datasets. Each dataset is formatted as follows:

n
string_1
...
string_n

Each dataset consists of n+1 lines.
The first line of each dataset contains an integer that indicates n (1 \leq n \leq 500).
The i-th line of the following n lines contains string_i, which consists of up to 10 English lowercase letters.

The end of the input is 0, and this should not be processed.

Output

Print either yes or
no in a line for each dataset, in the order of the input.
If all words in the dataset can be considered to be ordered lexicographically, print yes.
Otherwise, print no.

Sample Input

4
cba
cab
b
a
3
bca
ab
a
5
abc
acb
b
c
c
5
abc
acb
c
b
b
0

Output for the Sample Input

yes
no
yes
no

Solution in Python (judge verdict: Accepted)

graph = []

def init():
    global graph
    graph = [[False] * 26 + [True] for _ in xrange(27)]
    graph[26][26] = False

def atoi(c):#index
    if c == "#":
        return 26
    return ord(c) - ord("a")

def make_graph(L):
    global graph, count
    cur = 0
    L = [L[0]] + [L[i] for i in xrange(1, len(L)) if L[i] != L[i-1]]
    tmp = []
    for s1, s2 in zip(L, L[1:]):
        if s1[0] == s2[0]:
            tmp += [s1[1:], s2[1:]]
        else:
            if not tmp == []:
                make_graph(tmp)
            tmp = []
            graph[atoi(s2[0])][atoi(s1[0])] = True
    if not tmp == []:
        make_graph(tmp)

def check(start):
    stack = set([start])
    visited = [False] * 27
    while len(stack) != 0:
        cur = stack.pop()
        visited[cur] = True
        if graph[cur][start]:
            return False
        for i in xrange(27):
            if graph[cur][i] and not visited[i]:
                stack.add(i)
    return True

while True:
    n = input()
    if n == 0:
        break
    L = [raw_input() + "#" for _ in xrange(n)]
    init()
    make_graph(L)
    for i in xrange(27):
        if not check(i):
            print "no"
            break
    else:
        print "yes"