Given two words (start and end), and a dictionary, find the length of shortest transformation sequence 
from start to end, such that:

Only one letter can be changed at a time
Each intermediate word must exist in the dictionary
For example,

Given:
start = "hit"
end = "cog"
dict = ["hot","dot","dog","lot","log"]
As one shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog",
return its length 5.

Note:
Return 0 if there is no such transformation sequence.
All words have the same length.
All words contain only lowercase alphabetic characters.

class Solution:
    # @param start, a string
    # @param end, a string
    # @param dict, a set of string
    # @return an integer
    def ladderLength(self, start, end, dict):
        if start == end: return 0
        dict.add(end)

        visit = set([start])
        queue = [(start, 1)]
        chars = map(chr, range(ord('a'), ord('z')+1))

        while queue:
            head, layer = queue.pop(0)
            if head == end: return layer
            i = 0
            while i < len(head):
                for e in chars:
                    nb = head[:i] + e + head[i+1:]
                    if nb == end: return layer + 1
                    if nb in dict and not nb in visit:
                        queue.append((nb, layer + 1))
                        visit.add(nb)
                i += 1
        return 0