[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];
            }
        }
    }
};

python3，简单的双指针循环

class Solution:
    def strStr(self, haystack: str, needle: str) -> int:
        if len(needle) == 0:
            return 0
        for i in range(len(haystack)):
            if (i + len(needle)) <= len(haystack):
                flag = True
                for j in range(len(needle)):
                    if haystack[i + j] != needle[j]:
                        flag = False
                        break
                if flag:
                    return i
        return -1