*****(leetcode_string) Implement strStr() (KMP算法*****）

Implement strStr()

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Solution

Implement strStr().
Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
Update (2014-11-02):

The signature of the function had been updated to return the index instead of the pointer. If you still see your function signature returns a

char
 *

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String

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class Solution {
    void getNext(char *needle, int next[]){ //构造next数组
        int j = 0, k = -1;
        next[0]=-1;
        while(needle[j+1]!='\0'){
            if(k==-1 || needle[k]==needle[j]){
                ++j, ++k;
                if(needle[k]!=needle[j])
                    next[j]=k;
                else
                    next[j]=next[k];
            }else
                k=next[k];
        }
    }
public:
    int strStr(char *haystack, char *needle) {
        int *next = new int [strlen(needle)];
        getNext(needle, next);
        int i=0,j=0, index=0;
        while(haystack[i]!='\0'&&needle[j]!='\0'){
            if(haystack[i]==needle[j])
                i++,j++;
            else{
                //index+=j-next[j]; 可用来定位haystack中的位置， 不过用下面的i-strlen(needle) 更好理解
                if(next[j]==-1)
                    i++,j=0;
                else
                    j=next[j]; // 这条语句包含了两种情况， next[j]=0时，j=0  and  next[j]>=1时，j=next[j]; 
            }
        }
        if(needle[j]=='\0')
            return i-strlen(needle);
        else
            return -1;
    }
};