leetcode--Implement strStr()

1.题目描述

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

2.解法分析

KMP算法的一个应用而已，KMP算法见我的另一篇文章，以下是KMP算法应用在这个题目中。

class Solution { public: char *strStr(char *haystack, char *needle) { // Start typing your C/C++ solution below // DO NOT write int main() function if(needle==NULL)return needle; if(needle[0]=='\0')return haystack; int i=0;int j=0; vector<int>p=prepressing(needle); int len=p.size(); while(haystack[i]!='\0') { if(j==len) { return &haystack[i-len]; } if(haystack[i]==needle[j]) { i++;j++; } else { while(j!=0&&needle[j]!=haystack[i])j=p[j-1]; if(j==0&&needle[j]!=haystack[i])i++; } } if(j==len) { return &haystack[i-len]; } return NULL; } vector<int> prepressing(char *needle) { //ret[i]存放的是needle的前i+1个字符能再次利用的长度 vector<int> ret; ret.push_back(0); int i=1; while(needle[i]!='\0') { ret.push_back(0); int j=ret[i-1]; while(needle[i]!=needle[j]&&j!=0) { j=ret[j-1]; } if(needle[i]==needle[j])j++; ret[i]=j; i++; } return ret; } };