C - Find and Replace

#include <bits/stdc++.h>
 
using namespace std;
 
int main(){
    cin.tie(nullptr), ios::sync_with_stdio(false);
    int t;
    cin >> t;
    while(t--){
        int n;
        string s;
        cin >> n >> s;
        vector <int> chk1(26), chk2(26);
        for(int i = 0; i < n; i++){
            if(i & 1) chk1[s[i] - 'a']++;
            else chk2[s[i] - 'a']++;
        }
 
        bool flag = true;
        for(int i = 0; i < 26; i++) if(chk1[i] && chk2[i]) flag = false;
        cout << (flag ? "Yes\n" : "No\n");
    }
}