Substring with Concatenation of All Words——解题报告（窗口移动法）

短命女 2022-08-08 14:00 97阅读 0赞

【题目】

You are given a string, s, and a list of words, words, that are all of the same length. Find all starting indices of substring(s) in s that is a concatenation of each word in wordsexactly once and without any intervening characters.

For example, given:  
s: `"barfoothefoobarman"`  
words: `["foo", "bar"]`

You should return the indices: `[0,9]`.  
(order does not matter).

【分析】

本题目解法有两个，第一个是常规思路，第二个是窗口移动法，这个方法需要掌握。

解法一：先把words存在一个map中，key是单词字符串，value是出现的次数。然后逐个位置遍历字符串s（注意遍历结束位置不必到最后，剩余长度小于单词总长度即停止），判断其后面的和档次总长度相同的子串中的每个单词是否和words一样。如果一样，这push\_back进去；否则，遍历下一个字符。时间复杂度：O(LNw)，L是s的长度，N是words的个数，w是word的长度。

代码：

class Solution {
    public:
        vector<int> findSubstring(string s, vector<string>& words) {
            vector <int> res;
            int N = words.size(); // number of words
            if(N == 0)
                return res;
            int len = words[0].length();  // length of each word
            int strLen = s.length();  // length of string
            
            map<string, int> countWords;  // get the wordCount map
            for(int i = 0; i < N; i ++)
            {
                if(countWords.count(words[i]))
                    countWords[words[i]]++;
                else
                    countWords[words[i]] = 1;
            }
            
            map<string, int> counting;
            for(int i = 0; i <= strLen - len * N; i++)  // first loop
            {
                counting.clear();
                bool flag = true;
                for(int j = i; j < i + N*len; j += len)  // second loop
                {
                    string w = s.substr(j, len);
                    if(countWords.count(w) == 0)  //  if not exist in countWords, break directly
                    {
                        flag = false; 
                        break;
                    }
                    else
                    {
                        if(counting.count(w))  // if not new
                            counting[w]++;
                        else
                            counting[w] = 1;
                    }
                }
                if(!flag)
                    continue;
                else
                {
                    if(compare(counting, countWords))
                        res.push_back(i);
                    else
                        continue;
                }
            }
            
        }
        bool compare(map<string, int> counting, map<string, int> countWords)
        {
            map<string, int>::iterator iter;
            for(iter = countWords.begin(); iter != countWords.end(); iter++)
            {
                if(counting[iter->first] != iter->second)
                    return false;
            }
            return true;
        }
    };

**解法二：窗口移动法。**

**首先，初始化一个长度为0的窗口，定义头部是begin，尾部是tail。判断tail后面的一个单词，如果在words里面，则tail往后移动，即窗口伸长一个单词的量；如果tail后面的单词压根儿不在words里面，那么把begin后移到最后位置，重新初始化窗口，继续判断；如果tail后面的单词是之前出现过，不过words中没有这个单词的容量，那么begin后移到该单词第一次出现的位置，继续判断。这个方法的时间复杂度为O(Lw)。**

代码：

class Solution {
    public:
        vector<int> findSubstring(string S, vector<string> &L) {
            unordered_map<string, int>wordTimes;//L中单词出现的次数
            for(int i = 0; i < L.size(); i++)
                if(wordTimes.count(L[i]) == 0)
                    wordTimes.insert(make_pair(L[i], 1));
                else wordTimes[L[i]]++;
            int wordLen = L[0].size();
             
            vector<int> res;
            for(int i = 0; i < wordLen; i++)
            {//为了不遗漏从s的每一个位置开始的子串，第一层循环为单词的长度
                unordered_map<string, int>wordTimes2;//当前窗口中单词出现的次数
                int winStart = i, cnt = 0;//winStart为窗口起始位置,cnt为当前窗口中的单词数目
                for(int winEnd = i; winEnd <= (int)S.size()-wordLen; winEnd+=wordLen)
                {//窗口为[winStart,winEnd)
                    string word = S.substr(winEnd, wordLen);
                    if(wordTimes.find(word) != wordTimes.end())
                    {
                        if(wordTimes2.find(word) == wordTimes2.end())
                            wordTimes2[word] = 1;
                        else wordTimes2[word]++;
                         
                        if(wordTimes2[word] <= wordTimes[word])
                            cnt++;
                        else
                        {//当前的单词在L中，但是它已经在窗口中出现了相应的次数，不应该加入窗口
                         //此时，应该把窗口起始位置想左移动到，该单词第一次出现的位置的下一个单词位置
                            for(int k = winStart; ; k += wordLen)
                            {
                                string tmpstr = S.substr(k, wordLen);
                                wordTimes2[tmpstr]--;
                                if(tmpstr == word)
                                {
                                    winStart = k + wordLen;
                                    break;
                                }
                                cnt--;
                            }
                        }
                         
                        if(cnt == L.size())
                            res.push_back(winStart);
                    }
                    else
                    {//发现不在L中的单词
                        winStart = winEnd + wordLen;
                        wordTimes2.clear();
                        cnt = 0;
                    }
                }
            }
            return res;
        }
    };

另外还有一个代码，没有调试成功，后续修改ing。

class Solution {
    public:
        vector<int> findSubstring(string s, vector<string>& words) {
            vector <int> res;
            int N = words.size(); // number of words
            if(N == 0)
                return res;
            int len = words[0].length();  // length of each word
            int strLen = s.length();  // length of string
            
            map<string, int> countWords;  // get the wordCount map
            for(int i = 0; i < N; i ++)
            {
                if(countWords.count(words[i]))
                    countWords[words[i]]++;
                else
                    countWords[words[i]] = 1;
            }
            
            map<string, int> counting = countWords;
            int begin = 0, tail = begin;
            while(tail < strLen)
            {
                if(tail - begin + 1 == N * len)  // get results
                {
                    res.push_back(begin);
                    begin = tail + 1;
                    tail = begin;
                    counting = countWords;
                    continue;
                }
                
                string w = s.substr(tail, len);
                
                int kind = moveClass(w, counting);  // get the manner how to move the tail
                
                if(kind == 1)  // if w not in countWords
                {
                    begin = tail + 2;
                    tail = begin;
                    counting = countWords;
                    continue;
                }
                if(kind == 2) // if w in countWords now
                {
                    tail += len;
                    continue;
                }
                if(kind == 3)  // if w in countWords in past
                {
                    string tmp = s.substr(begin, tail - begin + 1);
                    begin = tmp.find(w) + 1;
                    tail = begin;
                    counting = countWords;
                    continue;
                }
            }
            return res; 
        }
        
        int moveClass(string w, map<string, int>& counting)
        {
            if(counting.count(w) == 0)
                return 1;
            else if(counting[w] >= 1)
            {
                counting[w]--;
                return 2;
            }
            else
                return 3;
        }
    };