Table of contents

• Problem
• Explanation
• Time complexity
• Solution

# Problem

https://atcoder.jp/contests/abc145/tasks/abc145_b

Given a string $S$, report if $S$ is twice repetitions of a string or not.

# Explanation

If the length of $S$ is odd, report No.
Otherwise, split $S$ in the middle into $S1$ and $S2$.
If $S1$ equals to $S2$, report Yes.

# Time complexity

Half of the length of $S$.

$O(N / 2)$

# Solution

// C++ 14
#include <iostream>
#include <string>
#include <vector>
#include <list>
#include <algorithm>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <unordered_map>
#include <math.h>

#define ll long long
#define Int int
#define loop(x, start, end) for(Int x = start; x < end; x++)
#define loopdown(x, start, end) for(int x = start; x > end; x--)
#define rep(n) for(int x = 0; x < n; x++)
#define span(a,x,y) a.begin()+x,a.begin()+y
#define span_all(a) a.begin(),a.end()
#define len(x) (x.size())
#define last(x) (*(x.end()-1))

using namespace std;

Int N;
string S;

void input() {
  cin >> N >> S;
}

void solve() {
  if (N % 2 == 1) {
    cout << "No" << endl;
    return;
  }

  if (S.substr(0, N/2) == S.substr(N/2)) cout << "Yes" << endl;
  else cout << "No" << endl;
}

int main(void) {
  input();
  solve();
  return 0;
}