Two String - use StringBuilder for output of console - look into StringBuilder vs string for concatenation

Twostring18.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
class Solution {
    static void Main(String[] args) {
        int t = Convert.ToInt32(Console.ReadLine());
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i < t; i++){
            bool[] chars = new bool[26];
            string str1 = Console.ReadLine();
            foreach(char c in str1.ToCharArray()){
                chars[c-'a'] = true;
            }
            string str2 = Console.ReadLine();
            bool found = false;
            foreach(char c in str2.ToCharArray()){
                if (chars[c-'a']){
                    sb.AppendLine("YES");
                    found = true;
                    break;
                }
            }
            if (!found){
                sb.AppendLine("NO");
            }
        }
        Console.WriteLine(sb.ToString());
    }
}