28. Implement strStr()

双指针遍历字符串

class Solution {
public:
    int strStr(string haystack, string needle) {
        if(needle.length()==0)
            return 0;
        int has=0,need=0,wide=(haystack.length()-needle.length()+1);
        while(has< wide)
        {
            if(haystack[has]==needle[need])
            {
                int len=1;
                has++;need++;
                for(;need<needle.length();)
                {
                    if(haystack[has]==needle[need])
                    {
                        has++;need++;len++;
                    }
                    else
                    {
                        need=0;
                        has=has-len+1;
                        break;
                    }
                }
                if(len==needle.length())
                    return has-needle.length();
                else
                    len=0;
            }
            else
            {
                has++;
            }
        }
        
        return -1;
    }
};