LeetCode 028 Implement strStr()

class Solution {
public:
    int strStr(char *haystack, char *needle) {
        // if needle is empty return the full string
        //if (!*needle) return (char*) haystack;
        if (needle[0] == ‘\0‘) return 0;
        
        int p1, p2, p1_advance = 0;
        
        for(p2 = 1; needle[p2]; p2++){
            p1_advance++;   //提前算好needle长度
        }
        
        //如果haystack[p1_advance]为空，则不必比较了！
        //长度都不够！
        for(p1 = 0; haystack[p1_advance]; p1_advance++){
            int p1_old = p1;
            p2 = 0;
            //都不为空，且相等，则一直比较
            while(haystack[p1] && needle[p2] && haystack[p1] == needle[p2]){
                p1++;
                p2++;
            }
            //如果比较到了needle结尾，则成功
            if(needle[p2] == ‘\0‘) 
                return p1_old;
            p1 = p1_old + 1;
        }
        return -1;
    }
};