[leetcode]Implement strStr()

真不容易。不靠debug写出来的。纸上分析的时候，写下i和j为两行，然后一个一个的写下改变的值，这样可以找到bug。

#include <cstring>

#include <malloc.h>

 

using namespace std;

 

class Solution {

public:

    char *strStr(char *haystack, char *needle) {

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        int needle_len = strlen(needle);

        int haystack_len = strlen(haystack);

        if (needle_len == 0) return haystack;

        if (haystack_len == 0) return 0;

 

        char* next = new char[needle_len];

        getNext(needle, next, needle_len);

 

        int i = 0;

        int j = 0;

         

        while (j < needle_len && i < haystack_len)

        {

            if (j == -1 || haystack[i] == needle[j])

            {

                i++;

                j++;

            }

            else

            {

                j = next[j];

            }

        }

 

        free(next);

        if (j == needle_len)

            return haystack+i-j;

        else

            return 0;

    }

 

    void getNext(char *needle, char * next, int len) {

        if (len == 0) return;

 

        int i = 0;

        int k = -1;

        next[0] = -1;

        while (i < len - 1)

        {

            if (k == -1|| needle[i] == needle[k]) {

                i++;

                k++;

                next[i] = k;

            }

            else {

                k = next[k];

            }

        }

    }

};