【leetcode】Word Ladder (hard) ★
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
.
思路:
找两点之间的最小距离,感觉只能用图,构建图的邻接矩阵,然后folyd 果断的超时了.....
int ladderLength1(string start, string end, unordered_set<string> &dict) {
if(start == end) return ;
int vexnum = dict.size() + ;
if(dict.find(start) != dict.end()) vexnum--;
if(dict.find(end) != dict.end()) vexnum--;
vector<string> s(vexnum); //记录graph中每行代表的单词
vector<vector<int>> graph(vexnum, vector<int>(vexnum, vexnum + )); //邻接矩阵
s[] = start;
s.back() = end;
int i = ;
unordered_set<string>::iterator it;
for(it = dict.begin(); it != dict.end(); it++)
{
if(*it != start && *it != end)
s[i++] = (*it);
} //记录有哪些单词可以通过一次变化相互转换
for(i = ; i < s.size(); i++)
{
string temp = s[i];
for(int j = ; j < start.size(); j++)
{
for(char c = 'a'; c <= 'z'; c++)
{
temp[j] = c;
if(dict.find(temp) != dict.end())
{
int edge = find(s.begin(), s.end(), temp) - s.begin();
if(edge == i)
{
graph[i][edge] = ;
graph[edge][i] = ;
}
else
{
graph[i][edge] = ;
graph[edge][i] = ;
}
}
}
}
} for(int k = ; k < graph.size(); k++)
{
for(int i = ; i < graph.size(); i++)
{
for(int j = ; j < graph.size(); j++)
{
if(graph[i][j] > graph[i][k] + graph[k][j])
{
graph[i][j] = graph[i][k] + graph[k][j];
}
}
}
} return (graph[][vexnum - ] == vexnum + ) ? : graph[][vexnum - ] + ;
}
来看大神的BFS算法,用dis记录每个点到start的距离。队列里开始只有start, 然后每次遇到新转化成的单词就进队列,更新距离。判断能否转化时用字符长度和26个字母,而不是对字典遍历,因为字典中单词的数量可能远大于26个。
int ladderLength(string start, string end, unordered_set<string> &dict)
{
unordered_map<string, int> dis;
queue<string> q;
dis[start] = ;
q.push(start);
while(!q.empty())
{
string word = q.front(); q.pop();
for(int i = ; i < start.size(); i++)
{
string temp = word;
for(char c = 'a'; c <= 'z'; c++)
{
temp[i] = c;
if(dict.count(temp) > && dis.count(temp) == )
{
dis[temp] = dis[word] + ;
q.push(temp);
}
}
}
}
if(dis.count(end) == ) return ;
return dis[end];
}
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