LeetCode: Implementing strStr()

KMP算法检查是否是子串，不太明白，先记下来。

    class Solution {  
    public:  
        char *strStr(char *haystack, char *needle) {  
            // Start typing your C/C++ solution below  
            // DO NOT write int main() function  
            int nLen = strlen(needle);  
            int* fail = new int[nLen];  
            memset(fail, -1, nLen*sizeof(int));  
            int i, j, k;  
            for (i = 1; i < nLen; ++i)  
            {  
                for (k = fail[i-1]; k >= 0 && needle[i] != needle[k+1]; k = fail[k]);  
                if (needle[k+1] == needle[i])  
                    fail[i] = k + 1;  
            }  
            i = j = 0;  
            while(haystack[i] && needle[j])  
            {  
                if (haystack[i] == needle[j])  
                {  
                    ++i;  
                    ++j;  
                }  
                else if(j == 0) ++i;  
                else j = fail[j-1] + 1;  
            }  
            delete fail;  
            if (needle[j]) return NULL;  
            else return haystack + i - j;  
        }  
    };