Leetcode#127 Word Ladder

 1 int ladderLength(string start, string end, unordered_set<string> &dict) {
 2         if (start == end)
 3             return 0;
 4             
 5         unordered_set<string> old;
 6         queue<string> layer;
 7         int len = 1;
 8         
 9         layer.push(start);
10         while (!layer.empty()) {
11             queue<string> nextLayer;
12             while (!layer.empty()) {
13                 string str = layer.front();
14                 layer.pop();
15                 if (str == end)
16                     return len;
17                 for (int i = 0; i < str.length(); i++) {
18                     for (char j = ‘a‘; j <= ‘z‘; j++) {
19                         string next = str;
20                         next[i] = j;
21                         if (old.find(next) == old.end() && dict.find(next) != dict.end()) {
22                             old.insert(next);
23                             nextLayer.push(next);
24                         }
25                     }
26                 }
27             }
28             len++;
29             layer = nextLayer;
30         }
31         
32         return 0;
33 }