leetcode -day10 Word Ladder I II

1、



Word Ladder

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

1. Only one letter can be changed at a time
2. 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.

分析：真的 一看这个题没有思路，想到每次改变一个字母，一直改下去，却不知道如何求最短。搜索得知，此题用到图的方法，广度优先遍历查找最短路径的方法，于是拿出《算法导论》看了一下图的广度优先遍历，大致思路是：用一个优先队列来保存访问过的点，每次取队列头元素，然后判断依次访问此元素的后继，然后将此元素出列，依次循环直到队列为空。此题相当于求最短路径，则需要判断到该结点的路径长度。

该题的思路：创建一个优先队列管理未遍历的结点，还有一个map，来保存遍历到的结点和距离。首先将start插入优先队列，然后开始循环，每次从优先队列取一个元素，并从其中删除，依次访问和此元素相差一个字母的值，查看此值是否在set中，如果不在或者在距离是否大于取出的元素的距离+1，则更新map，并插入此值，依次循环直到队列为空。

注意：其中有个很无聊的问题，看题目中转换次数为4，但是返回的长度为5，意思是加上了头尾。在执行过程中，当下面时，就显示2，不知道意思是如果start和end只有一个字母不同时，中间值无需字典中，感觉和题目要求中的Each intermediate word must exist in the dictionary不相符，很恶心。这样的话，应该在开始加一个判断start和end有几个字母不同的计算，如果为1，则直接返回2。

Input:"a", "c", ["a","b","c"]Output:3Expected:2

代码如下：

class Solution {public:int ladderLength(string start, string end, unordered_set<string> &dict) {if(start.empty()||end.empty()||start.length()!=end.length()||start == end){return 0;}if(dict.empty()){return 0;}unordered_map<string,int> pathMap;pathMap.insert(make_pair(start,1));queue<string> strQue;strQue.push(start);string tempStr;while(!strQue.empty()){tempStr = strQue.front();strQue.pop();for(int i=0; i<tempStr.size(); ++i){for(char j='a'; j<='z'; ++j){if(tempStr[i] == j){continue;}else{string replaceStr = tempStr;replaceStr[i] = j;if(dict.find(replaceStr)!=dict.end() || replaceStr == end){if(!pathMap[replaceStr] ||  pathMap[replaceStr] && pathMap[replaceStr] > pathMap[tempStr]+1){if(replaceStr == end && pathMap[tempStr]+1 < 2){continue;}pathMap[replaceStr] = pathMap[tempStr]+1;strQue.push(replaceStr);}}}}}}return pathMap[end];}};

2、Word Ladder II

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

1. Only one letter can be changed at a time
2. Each intermediate word must exist in the dictionary

For example,

Given:
start = "hit"
end = "cog"
dict = ["hot","dot","dog","lot","log"]

Return

  [    ["hit","hot","dot","dog","cog"],    ["hit","hot","lot","log","cog"]  ]

Note:

• All words have the same length.
• All words contain only lowercase alphabetic characters.

分析：这个题和上题很像，但是特别复杂，我采用上次的思路，添加了一个unordered_map<string,vector<string>> prevStrVec来保存前缀，前缀都是从start到key的最短路径上的前一个词，然后再根据end向前追溯一直到start，每一条路径就是保存的路径，思路感觉没有问题，本地跑几个小数据也是对的，就是一直出错。

首次出错为：Output Limit Exceeded

输出超过限制，就是可能中间出现的循环折叠，没找到问题，然后设置了长度递归限制，当路径超过最短路径值时返回，此时又出错：

Time Limit Exceeded

大数据时，显示超时

代码如下：超时的代码：

class Solution {public:vector<vector<string> > findLadders(string start, string end, unordered_set<string> &dict) {//vector<vector<string> > paths;if(start.empty()||end.empty()||start.length()!=end.length()||start == end){return paths;}if(dict.empty()){return paths;}swap(start,end);unordered_map<string,int> pathMap;pathMap.insert(make_pair(start,1));queue<string> strQue;strQue.push(start);string tempStr;unordered_map<string,vector<string>> prevStrVec;while(!strQue.empty()){tempStr = strQue.front();strQue.pop();for(int i=0; i<tempStr.size(); ++i){for(char j='a'; j<='z'; ++j){if(tempStr[i] == j){continue;}else{string replaceStr = tempStr;replaceStr[i] = j;if(dict.find(replaceStr)!=dict.end() || replaceStr == end){if(!pathMap[replaceStr] ||  pathMap[replaceStr] && pathMap[replaceStr] >= pathMap[tempStr]+1){if(replaceStr == end && pathMap[tempStr]+1 < 2){continue;}if(pathMap[replaceStr]  && pathMap[replaceStr] > pathMap[tempStr]+1){prevStrVec[replaceStr].clear();}prevStrVec[replaceStr].push_back(tempStr);pathMap[replaceStr] = pathMap[tempStr]+1;strQue.push(replaceStr);}}}}}}if(pathMap[end]){vector<string> tempVec;tempVec.push_back(end);int shortLength = pathMap[end];findPaths(end,prevStrVec,tempVec,shortLength);}return paths;}void findPaths(string& end,unordered_map<string,vector<string>> &prevStrVec,vector<string> &path,int shortLength){vector<string> preEndVec = prevStrVec[end];if(preEndVec.empty()){paths.push_back(path);return;}if(path.size()==shortLength){return;}for(int j=0; j<preEndVec.size(); ++j){string preStr = preEndVec[j];vector<string> tempPath = path;tempPath.push_back(preStr);findPaths(preStr,prevStrVec,tempPath,shortLength);}}vector<vector<string> > paths;};

改进，上述和第一题的区别是pathMap[replaceStr] 和pathMap[tempStr]+1 比较时，一个是>，一个是>=，后者将==的部分也放入队列了，这样时间复杂度高了好多，改进，==时不将数据放入队列，Accepted。

代码如下：

class Solution {public:vector<vector<string> > findLadders(string start, string end, unordered_set<string> &dict) {//vector<vector<string> > paths;if(start.empty()||end.empty()||start.length()!=end.length()||start == end){return paths;}if(dict.empty()){return paths;}swap(start,end);unordered_map<string,int> pathMap;pathMap.insert(make_pair(start,1));queue<string> strQue;strQue.push(start);string tempStr;unordered_map<string,vector<string>> prevStrVec;while(!strQue.empty()){tempStr = strQue.front();strQue.pop();for(int i=0; i<tempStr.size(); ++i){for(char j='a'; j<='z'; ++j){if(tempStr[i] == j){continue;}else{string replaceStr = tempStr;replaceStr[i] = j;if(dict.find(replaceStr)!=dict.end() || replaceStr == end){if(!pathMap[replaceStr] ||  pathMap[replaceStr] && pathMap[replaceStr] >= pathMap[tempStr]+1){if(replaceStr == end && pathMap[tempStr]+1 < 2){continue;}if(pathMap[replaceStr]  && pathMap[replaceStr] > pathMap[tempStr]+1){prevStrVec[replaceStr].clear();}else if(pathMap[replaceStr]  && pathMap[replaceStr] == pathMap[tempStr]+1){    prevStrVec[replaceStr].push_back(tempStr);    continue;}pathMap[replaceStr] = pathMap[tempStr]+1;strQue.push(replaceStr);prevStrVec[replaceStr].push_back(tempStr);}}}}}}if(pathMap[end]){vector<string> tempVec;tempVec.push_back(end);int shortLength = pathMap[end];findPaths(end,prevStrVec,tempVec,shortLength);}pathMap.clear();return paths;}void findPaths(string& end,unordered_map<string,vector<string>> &prevStrVec,vector<string> &path,int shortLength){vector<string> preEndVec = prevStrVec[end];if(preEndVec.empty()){paths.push_back(path);return;}    if(path.size()==shortLength){        return;    }for(int j=0; j<preEndVec.size(); ++j){string preStr = preEndVec[j];vector<string> tempPath = path;tempPath.push_back(preStr);findPaths(preStr,prevStrVec,tempPath,shortLength);}}vector<vector<string> > paths;};