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Problem solution · Python

ABC398 F — ABCBA

ABC398 F — ABCBA: a Python solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to KATO-Hiro AtCoder Solutions.

Technique
Direct simulation
Source
KATO-Hiro AtCoder Solutions
Length
64 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Direct simulation

For ABC398 F — ABCBA, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 64 lines of Python from the credited upstream file abc398_f.py.
  • The implementation visibly relies on sequence storage.
  • No explicit loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from KATO-Hiro AtCoder Solutions by KATO-Hiro and is used under the CC0-1.0 licence.

Open problem ↗Original source ↗
Full codeABC398 F — ABCBA · PythonPython
Use this to learn the idea, then write your own version.
# -*- coding: utf-8 -*- import typing  # See:# https://github.com/not522/ac-library-python/blob/master/atcoder/string.pydef z_algorithm(s: typing.Union[str, typing.List[int]]) -> typing.List[int]:    """    Reference:    D. Gusfield,    Algorithms on Strings, Trees, and Sequences: Computer Science and    Computational Biology    """     if isinstance(s, str):        s = [ord(c) for c in s]     n = len(s)     if n == 0:        return []     z = [0] * n    j = 0     for i in range(1, n):        z[i] = 0 if j + z[j] <= i else min(j + z[j] - i, z[i - j])         while i + z[i] < n and s[z[i]] == s[i + z[i]]:            z[i] += 1         if j + z[j] < i + z[i]:            j = i     z[0] = n     return z  def main():    import sys     input = sys.stdin.readline     s = input().rstrip()    n = len(s)    t = s[::-1] + s    z = z_algorithm(t)     for i in range(n):        if z[n + i] != n - i:            continue         ans = s        ans += t[n - i : n]        print(ans)         break  if __name__ == "__main__":    main()

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