leetcode 28:实现strStr()

int strStr(string haystack, string needle) {
    if(needle.size()==0)
        return 0;
    for(int i=0;i<haystack.size();i++){
        int flag=0;
        if(needle.size()>(haystack.size()-i))return -1;
        if(haystack[i]==needle[0]){
            int j=1;
            int k=i+1;
            for(;j<needle.size()&&k<haystack.size()&&k-i<=needle.size();j++,k++){
                if(needle[j]!=haystack[k]){
                    flag=1;
                    break;
                }
            }
            if(flag==0&&k-i==needle.size())
                return i;
        }
    }
    return -1;
}