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Problem solution · Go

Codeforces 449B — Jzzhu and Cities

Codeforces 449B — Jzzhu and Cities: a Go solution using stack-based processing. Learn the idea, check the complexity, and read the full code, with credit to EndlessCheng Codeforces Go.

Technique
Stack-based processing
Source
EndlessCheng Codeforces Go
Length
66 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

Stack-based processing

For Codeforces 449B — Jzzhu and Cities, the implementation keeps unresolved items in last-in, first-out order, often to match boundaries, parse structure, or maintain monotonic candidates.

  1. Define what every stack entry represents.
  2. Pop entries once the current item resolves or invalidates them.
  3. Push the current item with only the information later steps need.

Code notes

  • 66 lines of Go from the credited upstream file 449B.go.
  • The implementation visibly relies on sequence storage.
  • No explicit loop blocks detected.

Complexity

If each item is pushed and popped at most once, the stack work is linear.

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 449B — Jzzhu and Cities · GoGo
Use this to learn the idea, then write your own version.
package main import (	"bufio"	"container/heap"	. "fmt"	"io") // github.com/EndlessCheng/codeforces-gotype pr49 struct {	v   int	d   int64	isT bool}type hp49 []pr49 func (h hp49) Len() int              { return len(h) }func (h hp49) Less(i, j int) bool    { return h[i].d < h[j].d || h[i].d == h[j].d && !h[i].isT } // 同等距离下优先选不是火车的路func (h hp49) Swap(i, j int)         { h[i], h[j] = h[j], h[i] }func (h *hp49) Push(v interface{})   { *h = append(*h, v.(pr49)) }func (h *hp49) Pop() (v interface{}) { a := *h; *h, v = a[:len(a)-1], a[len(a)-1]; return }func (h *hp49) push(v pr49)          { heap.Push(h, v) }func (h *hp49) pop() pr49            { return heap.Pop(h).(pr49) } func CF449B(_r io.Reader, out io.Writer) {	in := bufio.NewReader(_r)	var n, m, k, v, w, wt, ans int	Fscan(in, &n, &m, &k)	type nb struct {		to, wt int		isT    bool	}	g := make([][]nb, n)	for ; m > 0; m-- {		Fscan(in, &v, &w, &wt)		v--		w--		g[v] = append(g[v], nb{w, wt, false})		g[w] = append(g[w], nb{v, wt, false})	}	for ; k > 0; k-- {		Fscan(in, &w, &wt)		g[0] = append(g[0], nb{w - 1, wt, true})	}	vis := make([]bool, n)	q := hp49{{}}	for len(q) > 0 {		p := q.pop()		v := p.v		if vis[v] {			if p.isT {				ans++			}			continue		}		vis[v] = true		for _, e := range g[v] {			q.push(pr49{e.to, p.d + int64(e.wt), e.isT})		}	}	Fprint(out, ans)} //func main() { CF449B(os.Stdin, os.Stdout) }

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