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Problem solution · Python

ABC323 E — Playlist

ABC323 E — Playlist: a Python solution using dynamic programming. Learn the idea, check the complexity, and read the full code, with credit to KATO-Hiro AtCoder Solutions.

Technique
Dynamic programming
Source
KATO-Hiro AtCoder Solutions
Length
42 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

Dynamic programming

For ABC323 E — Playlist, the implementation records answers for smaller states and reuses them to build the requested result without repeating work.

  1. Define precisely what one DP state represents.
  2. Establish the base cases before transitions are evaluated.
  3. Process states in dependency order and combine only already-known values.

Code notes

  • 42 lines of Python from the credited upstream file abc323_e.py.
  • The implementation visibly relies on sequence storage, ordered lookup, cached states.
  • No explicit loop blocks detected.

Complexity

Multiply the number of reachable states by the work performed for each transition, then include the stored state table in memory usage.

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 codeABC323 E — Playlist · PythonPython
Use this to learn the idea, then write your own version.
# -*- coding: utf-8 -*-  def main():    import sys     input = sys.stdin.readline     n, x = map(int, input().split())    t = list(map(int, input().split()))    mod = 998244353    inv = pow(n, mod - 2, mod)  # 1 / p     # ランダムに何曲か選ぶ + 最後に曲1を選んで、x秒より大きいときが答え    # dp[i]: 再生がi秒で終わる確率    dp = [0] * (x + 1)    dp[0] = 1     # 各時点において、すべての曲を試す    for i in range(x + 1):        for tj in t:            ni = i + tj             if ni > x:                continue             dp[ni] += dp[i] * inv            dp[ni] %= mod     ans = 0     for i, dp_i in enumerate(dp):        if i + t[0] > x:            ans += dp_i * inv            ans %= mod     print(ans)  if __name__ == "__main__":    main()

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