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Problem solution · Go

Codeforces 1051E — Vasya and Big Integers

Codeforces 1051E — Vasya and Big Integers: a Go solution using sliding window or two pointers. Learn the idea, check the complexity, and read the full code, with credit to EndlessCheng Codeforces Go.

Technique
Sliding window or two pointers
Source
EndlessCheng Codeforces Go
Length
87 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

Sliding window or two pointers

For Codeforces 1051E — Vasya and Big Integers, the implementation maintains a moving interval and updates only the information that enters or leaves the window.

  1. Choose the invariant that makes a window valid or useful.
  2. Advance the right boundary and add the new element.
  3. Move the left boundary only as needed while maintaining the invariant and updating the answer.

Code notes

  • 87 lines of Go from the credited upstream file 1051E.go.
  • The implementation visibly relies on sequence storage.
  • No explicit loop blocks detected.

Complexity

Confirm that neither pointer moves backwards; if so, the scan is usually linear apart from the window’s data-structure operations.

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

Source

Code and credit

This code comes from EndlessCheng Codeforces Go by Σndless (EndlessCheng) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeCodeforces 1051E — Vasya and Big Integers · GoGo
Use this to learn the idea, then write your own version.
package main import (	"bufio"	. "fmt"	"io") // https://space.bilibili.com/206214func calcZ51(s string) []int {	n := len(s)	z := make([]int, n)	for i, l, r := 1, 0, 0; i < n; i++ {		if i <= r {			z[i] = min(z[i-l], r-i+1)		}		for i+z[i] < n && s[z[i]] == s[i+z[i]] {			l, r = i, i+z[i]			z[i]++		}	}	z[0] = n	return z} func cf1051E(_r io.Reader, out io.Writer) {	in := bufio.NewReader(_r)	const mod = 998244353	var S, L, R string	Fscan(in, &S, &L, &R) 	n, nl, nr := len(S), len(L), len(R)	if nl > n {		Fprint(out, 0)		return	}	zl := calcZ51(L + S)	zr := calcZ51(R + S) 	sum := make([]int, n+2)	sum[1] = 1	pre := 0	for i := 1; i <= n; i++ {		sum[i+1] = sum[i]		if i < nl {			pre = 0			continue		}				// [l,r]		l, r := i-nr, i-nl		if lcp := zl[nl+r]; lcp < nl && S[r+lcp] < L[lcp] {			r--			if r < 0 {				pre = 0				continue			}		}		if l < 0 {			l = 0		} else {			if lcp := zr[nr+l]; lcp < nr && S[l+lcp] > R[lcp] {				l++			}		}		if l > r {			pre = 0			continue		}				f := 0		if L == "0" && S[i-1] == '0' {			f = pre			r--		}		f = (f + sum[r+1] - sum[l]) % mod		pre = f		if i < n && S[i] == '0' {			f = 0		}		sum[i+1] += f	}	Fprint(out, pre)} //func main() { cf1051E(os.Stdin, os.Stdout) }

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