leetcode 28. Implement strStr()

题目

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

思路

实现kmp算法

代码实现

class Solution {    public int strStr(String haystack, String needle) {        //检查输入参数        if(haystack==null){            return -1;        }        if(needle == null||needle.length()==0){            return 0;        }        int [] next=getNext(needle);        int i=0;        int j=0;        int slen=haystack.length();        int plen=needle.length();        while(i<slen && j< plen){            if(j==-1|| haystack.charAt(i)==needle.charAt(j)){                ++i;                ++j;            }else {                j=next[j];            }        }        if(j == plen){            return i-j;        }else {             return -1;        }    }    int [] getNext(String needle){        int len=needle.length();        int[] next=new int[len];        next[0]=-1;        int i=-1;        int j=0;        while(j<len-1){                //p[i]表示前缀，p[j]表示后缀                //next数组存储的后缀中相应位置对应的前缀next[0] 已经有了对应的值.因此从i=0,j=1开始.next[1]=0                if(i==-1||needle.charAt(i) == needle.charAt(j)){                   ++i;                   ++j;                   next[j]=i;               }else {                   i=next[i];               }        }        return next;    } }