Two string - using 'z'-'a' in array declaration, use one array size of 26, second one is to use a string.

TwoString11.cs

using System;
using System.Collections.Generic;
using System.IO;
class Solution {
    static void Main(String[] args) {
        int lines = Convert.ToInt32(Console.ReadLine());
        while(lines-- > 0)
        {
            bool[] founded = new bool['z' - 'a' + 1];
            bool found = false;
            char[] firstWord = Console.ReadLine().ToCharArray();   
            foreach (char c in firstWord)
                founded[c - 'a'] = true;
            char[] secondWord = Console.ReadLine().ToCharArray();
            foreach (char c in secondWord)
            {
                if (founded[c - 'a']) 
                {
                    found = true;
                    break;
                }
            }
            Console.WriteLine(found ? "YES" : "NO");
        }
    }
}