func_code_string: #include <bits/stdc++.h>   using namespace std;   #define int long long   #define pii pair<int, int>   int32_t mod = 1e9 + 7;      void solveCase()   {       int n = 0, m = 0;       cin >> n >> m;          vector<vector<int>> mp(m, vector<int>(26, 0));       for (int i = 0; i < n; i++)       {           string s;           cin >> s;           for (int i = 0; i < m; i++)               mp[i][s[i] -  a ]++;       }       for (int i = 0; i < n - 1; i++)       {           string s;           cin >> s;           for (int i = 0; i < m; i++)               mp[i][s[i] -  a ]--;       }          for (int i = 0; i < m; i++)           for (int x = 0; x < 26; x++)               if (mp[i][x])                   cout << char( a  + x);       cout <<   n ;   }      int32_t main()   {       ios::sync_with_stdio(false), cin.tie(NULL);       int t = 0;       cin >> t;       while (t--)           solveCase();   }