028 Implement StrStr()

LeetCode Question 28 Implementing StrStr()

Implement the strStr() function.
Given a haystack string and a needle string, find
the first position (0-based) in the haystack string where the needle string occurs. Returns -1 if not present.
Example 1:
Input: haystack = "hello", needle = "ll"
Output: 2
Example 2:
Input: haystack = "aaaaa", needle = "bba"
Output: -1

This topic needs to pay attention to some edge data, which needs special attention!

Java

    public int strStr(String haystack, String needle) {
        if (needle.isEmpty())
            return 0;
        int i = 0, j = 0;
        for (i = 0; i <= haystack.length() - needle.length(); i++) {
            for (j = 0; j < needle.length(); j++) {
                if (haystack.charAt(i + j) != needle.charAt(j))
                    break;
            }
            if (j == needle.length())
                return i;
        }
        return -1;
    }

C++

class Solution
{
public:
    int strStr(string haystack, string needle)
    {
        if(needle.length()==0)return 0;
        int i,j;
        int len_1 = haystack.length();
        int len_2 = needle.length();
        for(i=0; i<=(len_1-len_2); i++)
        {
            for(j=0; j<needle.length(); j++)
            {
                if(haystack[i+j]!=needle[j])
                    break;
            }
            if(j==needle.length())
                return i;
        }
        return -1;
    }
};

I encountered a bug here, and it took a long time to find it, which is very strange:

int main()
{
    Solution s;
    string a="";
    string b = "a";
    cout<<a.length()<<endl;
    cout<<b.length()<<endl;
    cout<<a.length()-b.length()<<endl;
    return 0;
}

The output is:

0
1
4294967295

Shouldn't the third output be: -1? So I tried to use the a.size() function again, and the result was the same. In fact, the size() function in string is the same as the length() function.

If anyone knows where I am wrong, please correct me!