Implement strStr().

Returns a pointer to the first occurrence of needle in haystack,or null if needle is not part of haystack.

class Solution {
public:
    char *strStr(char *haystack, char *needle) {
        int hayLen = strlen(haystack);
        int needleLen = strlen(needle);
        
        int next[needleLen];
        get_nextval(needle, next);
        
        if (hayLen == 0 && needleLen != 0)
            return NULL;
        if (hayLen == 0 || needleLen == 0) 
            return haystack;
        int i = 0, j = 0;  
        while(i < hayLen && j < needleLen)
        {
            if(haystack[i] != needle[j])
            {
                 if(next[j]!=-1)
                    j=next[j];
                 else
                 {
                    j=0;
                    i++;
                 }
            }
            else
            {
                i++;j++;
            }
        }
        if(j==needleLen) return haystack+i-needleLen;
        return NULL;
    }
    void get_nextval(const char *T, int next[])
    {
       int j = 0, k = -1;
       next[0] = -1;
       while ( T[j] != '\0' )
       {
           if (k == -1 || T[j] == T[k]) //字符相等，或者是字符不相等且没有上一个匹配位置了
           {
                ++j; ++k;
                if (T[j]!=T[k]) //前一个字符相等，这个字符不相等
                    next[j] = k; //下一次从k开始判断（因为k-1都已经匹配相等）
                else //相等，下一次开始判断的位置和k位置下次开始判断的位置相同
                    next[j] = next[k];
            }
            else //字符不相等，模式串移到上一个匹配位置继续匹配
                 k = next[k];
       }
    }
};

class Solution {
public:
    char *strStr(char *haystack, char *needle) {
        int hayLen = strlen(haystack);
        int needleLen = strlen(needle);
      
        if (hayLen == 0 && needleLen != 0)
            return NULL;
        if (hayLen == 0 || needleLen == 0) 
            return haystack;
            
        for(int i = 0; i< hayLen; i++)
        {
            int j = 0;
            for(; j< needleLen; j++)
            {
                if(haystack[j+i] != needle[j]) break;
            }
            if (j == needleLen) return &haystack[i];
        }
        return NULL;  
    }
};