Codeforces Beta Round 91 (Div. 1 Only)

Solutions for Codeforces Beta Round 91 (Div. 1 Only) (contest 121). 5/5 problems verified against sample I/O. Difficulty range: 1100-2500.

5 items

Codeforces Beta Round 91 (Div. 1 Only)

Type: Div. 1 | Problems: 5 | Verified: 5/5 | Rating range: 1100-2500 | Time: 9m 56s

Problem Name Rating Tags Solve Time Verified
A Lucky Sum 1100 implementation 1m 36s ✓
B Lucky Transformation 1500 strings 2m 26s ✓
C Lucky Permutation 1900 brute-force, combinatorics, number-theory 1m 34s ✓
D Lucky Segments 2500 binary-search, implementation, two-pointers 1m 59s ✓
E Lucky Array 2400 data-structures 2m 21s ✓
Practice › Codeforces Solutions › Codeforces Beta Round 91 (Div. 1 Only) ›

CF 121A - Lucky Sum

We need to evaluate a sum over an interval [l, r]. For every integer x in that range, we compute next(x), where next(x) means the smallest lucky number greater than or equal to x. A lucky number is a positive integer whose decimal digits are only 4 and 7.
codeforces competitive-programming implementation May 28, 2026
Practice › Codeforces Solutions › Codeforces Beta Round 91 (Div. 1 Only) ›

CF 121B - Lucky Transformation

We are given a decimal string and an operation count k. Each operation looks for the leftmost occurrence of the substring "47". Suppose the substring "47" starts at position x using 1-based indexing. If x is odd, we replace both digits with '4', so "47" becomes "44".
codeforces competitive-programming strings May 28, 2026
Practice › Codeforces Solutions › Codeforces Beta Round 91 (Div. 1 Only) ›

CF 121C - Lucky Permutation

We are asked to examine permutations of numbers from 1 to n and focus on "lucky numbers" - integers that contain only the digits 4 and 7.
codeforces competitive-programming brute-force combinatorics number-theory May 28, 2026
Practice › Codeforces Solutions › Codeforces Beta Round 91 (Div. 1 Only) ›

CF 121D - Lucky Segments

We are asked to find the maximum number of "full lucky numbers" that can appear after adjusting segments of numbers. A lucky number is any positive integer whose digits consist only of 4 and 7. Each segment is a range [li, ri] on the number line.
codeforces competitive-programming binary-search implementation two-pointers May 28, 2026
Practice › Codeforces Solutions › Codeforces Beta Round 91 (Div. 1 Only) ›

CF 121E - Lucky Array

We have an array whose values are always at most 10000. Two operations must be processed online. The first operation adds a value d to every element inside a segment [l, r].
codeforces competitive-programming data-structures May 28, 2026