提出 #64073926

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ソースコード 拡げる

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
constexpr int MAXN = 1'000'007;
constexpr int INF = 2'000'000'000;
constexpr ll INFF = 1'000'000'000'000'000'000LL;
constexpr ll MOD = 998'244'353;
#define mkp make_pair
#define F first
#define S second
#define pb emplace_back
#define sz(v) ((int)(v).size())
#define all(v) (v).begin(), (v).end()

int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);

  auto manacher = [&](string s) {
    // z[i] => longest odd palindrome centered at i is
    //         s[i - z[i] ... i + z[i]]
    // to get all palindromes (including even length),
    // insert a '#' between each s[i] and s[i + 1]
    int n = s.size();
    vector<int> z;
    z.resize(n);
    z[0] = 0;
    for (int b = 0, i = 1; i < n; i++) {
      if (z[b] + b >= i)
        z[i] = min(z[2 * b - i], b + z[b] - i);
      else
        z[i] = 0;
      while (i + z[i] + 1 < n && i - z[i] - 1 >= 0 &&
             s[i + z[i] + 1] == s[i - z[i] - 1])
        z[i]++;
      if (z[i] + i > z[b] + b)
        b = i;
    }
    return z;
  };

  string s;
  cin >> s;
  int n = sz(s);
  string ss;
  ss.push_back(s[0]);
  for (int i = 1; i < sz(s); i++)
    ss.push_back('*'), ss.push_back(s[i]);
  //   cout << ss << endl;
  auto z = manacher(ss);

  for (int i = sz(ss) - n; i <= sz(ss) - 1; i++) {
    if (z[i] == sz(ss) - i - 1) {
      cout << s;
      for (int j = i - z[i] - 1; j >= 0; j--) {
        if (ss[j] != '*')
          cout << ss[j];
      }
      cout << '\n';
      break;
    }
  }
}
/*
- How to come up with solutions? (https://hackmd.io/-3cIVAFSQMC3iJTh9EpszA)
  - Play with some small examples.
  - Make observations (or random guesses) by intuition.
    - Try to find counterexamples of the "observations."
  - Find the characteristics of an optimal solution.
  - Try to solve simple cases.
  - Brute force and print out the results.
  - Pick a method (greedy, dp, d&c, ...)
    - But DO NOT force algos on problems!
  - IF YOU'RE STUCK, TRY SOMETHING ELSE! Make new observations!

- Before writing the solution:
  - check TL/ML/correctness of samples/implementation details!

- Pre-submit:
  - Did you make a typo when copying a template?
  - Test more cases if unsure.
    - Write a naive solution and check small cases.
  - Submit the correct file.

- Debugging:
  - General Debugging:
    - Read the whole problem again.
    - Think over the algorithm again.
    - Go to the toilet.

  - Wrong Answer:
    - Any possible overflows?
      - > `__int128` ?
      - Try `-ftrapv` or `#pragma GCC optimize("trapv")`
    - Floating point errors?
      - > `long double` ?
      - turn off math optimizations
      - check for `==`, `>=`, `acos(1.000000001)`, etc.
    - Did you forget to sort or unique?
    - Generate large and worst "corner" cases.
    - Check your `m` / `n`, `i` / `j`,  `x` / `y` and `<` / `>`.
    - Are everything initialized or reset properly?
    - Are you sure about the STL thing you are using?
      - Read cppreference.
    - Print everything and run it on pen and paper.

  - Time Limit Exceeded:
    - Calculate your time complexity again.
    - Does the program actually end?
      - Check for `while(q.size())` etc.
    - Test the largest cases locally.
    - Did you do unnecessary stuff?
      - e.g. pass vectors by value
      - e.g. `memset` for every test case
    - Is your constant factor reasonable?

  - Runtime Error:
    - Check memory usage.
      - Forget to clear or destroy stuff?
      - > `vector::shrink_to_fit()`
    - Stack overflow?
    - Bad pointer / array access?
      - Try `-fsanitize=address`
    - Division by zero? NaN's?
*/

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