Longest Common Prefix

Input: strs = ["flower","flow","flight"]
Output: "fl"

Input: strs = ["dog","racecar","car"]
Output: ""
Explanation: There is no common prefix among the input strings.

public string LongestCommonPrefix(string[] strs)
{
    if (strs.Length == 0) return null;

    string res = string.Empty;
    string str = strs[0];

    foreach (var s in strs)
    {
        if (s.Length < str.Length)
            str = s;
    }

    int len = strs.Length;

    for (int i = 0; i < str.Length; i++)
    {
        for (int j = 0; j < len; j++)
        {
            if (str[i] == strs[j][i])
                continue;
            else
                return res;
        }

        res += str[i];
    }

    return res;
}