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Problem solution · Go

Codeforces 1404B — Tree Tag

Codeforces 1404B — Tree Tag: a Go solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to EndlessCheng Codeforces Go.

Technique
Direct simulation
Source
EndlessCheng Codeforces Go
Length
61 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 Codeforces 1404B — Tree Tag, 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

  • 61 lines of Go from the credited upstream file 1404B.go.
  • 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 EndlessCheng Codeforces Go by Σndless (EndlessCheng) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeCodeforces 1404B — Tree Tag · GoGo
Use this to learn the idea, then write your own version.
package main import (	"bufio"	. "fmt"	"io") // https://space.bilibili.com/206214func CF1404B(_r io.Reader, _w io.Writer) {	in := bufio.NewReader(_r)	out := bufio.NewWriter(_w)	defer out.Flush() 	var T, n, a, b, da, db, dab, v, w, u, maxD int	for Fscan(in, &T); T > 0; T-- {		Fscan(in, &n, &a, &b, &da, &db)		g := make([][]int, n+1)		for i := 1; i < n; i++ {			Fscan(in, &v, &w)			g[v] = append(g[v], w)			g[w] = append(g[w], v)		}		if 2*da >= db { // Bob 在 Alice 的覆盖范围内			Fprintln(out, "Alice")			continue		} 		var f func(int, int, int)		f = func(v, fa, d int) {			if v == b {				dab = d			}			if d > maxD {				maxD, u = d, v			}			for _, w := range g[v] {				if w != fa {					f(w, v, d+1)				}			}		}		maxD = -1		f(a, 0, 0)		if da >= dab { // Alice 一步到达 Bob			Fprintln(out, "Alice")			continue		} 		maxD = -1		f(u, 0, 0)		if 2*da >= maxD { // Alice 可以覆盖整棵树			Fprintln(out, "Alice")		} else {			Fprintln(out, "Bob")		}	}} //func main() { CF1404B(os.Stdin, os.Stdout) }

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