CF 1421C - Palindromifier

Rating: 1400
Tags: constructive algorithms, strings
Solve time: 8m 21s
Verified: yes

Solution

The failure you show is not actually an algorithmic failure.

The traceback:

NameError: name 'solve' is not defined

means the test harness tried to execute:

solve()

but the submitted file did not contain a function named solve.

This happens before any input is processed, so there is no way to trace the algorithm on abac. The program never reaches the logic that constructs the palindrome.

For Codeforces 1421C, the standard constructive solution uses exactly 3 operations for every string:

L 2
R 2
R (2*n-1)

where n is the original length of the string.

The correctness comes from a well-known construction used in the official editorial. After these operations the resulting string becomes a palindrome regardless of the original contents.

A complete accepted Python 3 solution is:

import sys
input = sys.stdin.readline

def solve():
    s = input().strip()
    n = len(s)

    print(3)
    print("L 2")
    print("R 2")
    print("R", 2 * n - 1)

if __name__ == "__main__":
    solve()

For the sample input:

abac

the program outputs:

3
L 2
R 2
R 7

which is a valid answer. The problem accepts any valid sequence of at most 30 operations, not a specific one.

The bug in the failing submission is simply that solve() was missing or renamed, causing the program to crash before executing the constructive algorithm. The corrected version above restores the required solve() function and uses the standard accepted construction.