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,确实能够大大提高效率。code例如以下:
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; else N[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; } };