leetcode——Implement strStr() 实现字符串匹配函数（AC）

Implement strStr().

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

这个题考查的是KMP算法，先求特征向量，然后再进行匹配，确实可以大大提高效率。code如下：

class Solution {public:    char *strStr(char *haystack, char *needle) {        if(strlen(haystack)==0&&strlen(needle)==0)            return haystack;        if(strlen(haystack)==0&&strlen(needle)!=0)return NULL;if(strlen(needle)==0)    return haystack;int m=strlen(needle);        int *N = new int[m];N[0]=0;int i,j,k;for(i=1;i<m;i++){k=N[i-1];while(k>0 && needle[i]!=needle[k]){k=N[k-1];}if(needle[i]==needle[k])N[i]=k+1;elseN[i]=0;}j=0;for(i=0;i<strlen(haystack);i++){while(j>0 && needle[j]!=haystack[i])j=N[j-1];if(needle[j]==haystack[i])j++;if(j==strlen(needle))return haystack+i-j+1;}return NULL;    }};