402-字符串（题目：剑指Offer58-II.左旋转字符串、 28. 实现 strStr()、459.重复的子字符串）

题目：剑指Offer58-II.左旋转字符串

class Solution {
    
public:
    string reverseLeftWords(string s, int n) {
    
        string s1(s.begin(), s.begin()+n);
        string s2(s.begin()+n, s.end());
        return s2+s1;
    }
};

28. 实现 strStr()

class Solution {
    

public:
    void get_next(vector<int>& next, const string& str)
    {
    
        int j = 0;
        int k = -1;
        next[j] = k;

        while (j < str.size() - 1)
        {
    
            if (-1 == k || str[j] == str[k])
            {
    
                j++;
                k++;
                next[j] = k;
            }
            else
            {
    
                k = next[k];
            }
        }
    }


public:
    int strStr(string haystack, string needle) {
    

        vector<int> next(needle.size(), 0);

        get_next(next, needle);

        int i = 0;
        int j = 0;

        int size1 = haystack.size();
        int size2 = needle.size();

        while (i < size1 && j < size2)
        {
    
            if (-1 == j || haystack[i] == needle[j])
            {
    
                i++;
                j++;
            }
            else
            {
    
                j = next[j];
            }
        }

        if (j == needle.size())
        {
    
            return i - j;
        }
        else
        {
    
            return -1;
        }
    }
};

459.重复的子字符串

自己加自己，掐头去尾找自己！

class Solution {
    
public:
    bool repeatedSubstringPattern(string s) {
    
        string s1(s+s);
        string s2 = s1.substr(1, 2*s.size() - 2);

        if(s2.find(s) == -1)
        {
    
            return false;
        }

        return true;
    }
};