Leetcode: Word Ladder @Python

class Solution:
    # @param start, a string
    # @param end, a string
    # @param dict, a set of string
    # @return an integer
    def ladderLength(self, start, end, dic):
        alph = [chr(c) for c in range(97, 123)]
        dic.add(end)
        n,m=len(dic),len(start)
        que=collections.deque([[start,1]])
        while que:
            crt=que.popleft()
            crtword,crtlen=crt[0],crt[1]
            if crtword==end: return crtlen
            for i in range(m):
                s1=crtword[:i]
                s2=crtword[i+1:]
                for c in alph:
                    nextword=s1+c+s2
                    if nextword in dic and c!=crtword[i]:
                        dic.remove(nextword)
                        que.append([nextword,crtlen+1])
        return 0