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Problem solution · Go

Codeforces 786B — Legacy

Codeforces 786B — Legacy: 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
119 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 786B — Legacy, 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

  • 119 lines of Go from the credited upstream file 786B.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 786B — Legacy · 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 pair86 struct {	v   int	dis int64}type hp86 []pair86 func (h hp86) Len() int              { return len(h) }func (h hp86) Less(i, j int) bool    { return h[i].dis < h[j].dis }func (h hp86) Swap(i, j int)         { h[i], h[j] = h[j], h[i] }func (h *hp86) Push(v interface{})   { *h = append(*h, v.(pair86)) }func (h *hp86) Pop() (v interface{}) { a := *h; *h, v = a[:len(a)-1], a[len(a)-1]; return }func (h *hp86) push(v pair86)        { heap.Push(h, v) }func (h *hp86) pop() pair86          { return heap.Pop(h).(pair86) } func CF786B(_r io.Reader, _w io.Writer) {	in := bufio.NewReader(_r)	out := bufio.NewWriter(_w)	defer out.Flush() 	var n, m, st, tp, v, w, wt, l, r int	Fscan(in, &n, &m, &st)	d := 4 * n	type nb struct{ to, wt int }	g := make([][]nb, 2*d)	leaf := make([]int, n+1) 	type seg []struct{ l, r int }	t := make(seg, d)	var build func(o, l, r int)	build = func(o, l, r int) {		t[o].l, t[o].r = l, r		if l == r {			leaf[l] = o			g[o] = append(g[o], nb{o + d, 0})			g[o+d] = append(g[o+d], nb{o, 0})			return		}		lo, ro := o<<1, o<<1|1		g[o] = append(g[o], nb{lo, 0}, nb{ro, 0})		g[lo+d] = append(g[lo+d], nb{o + d, 0})		g[ro+d] = append(g[ro+d], nb{o + d, 0})		m := (l + r) >> 1		build(lo, l, m)		build(ro, m+1, r)	}	build(1, 1, n) 	var conn func(int)	conn = func(o int) {		if l <= t[o].l && t[o].r <= r {			if tp == 2 {				g[v] = append(g[v], nb{o, wt})			} else {				g[o+d] = append(g[o+d], nb{v, wt})			}			return		}		m := (t[o].l + t[o].r) >> 1		if l <= m {			conn(o << 1)		}		if m < r {			conn(o<<1 | 1)		}	}	for ; m > 0; m-- {		if Fscan(in, &tp); tp == 1 {			Fscan(in, &v, &w, &wt)			g[leaf[v]] = append(g[leaf[v]], nb{leaf[w], wt})		} else {			Fscan(in, &v, &l, &r, &wt)			v = leaf[v]			conn(1)		}	} 	const inf int64 = 1e18	dist := make([]int64, 2*d)	for i := range dist {		dist[i] = inf	}	st = leaf[st]	dist[st] = 0	q := hp86{{st, 0}}	for len(q) > 0 {		p := q.pop()		v := p.v		if dist[v] < p.dis {			continue		}		for _, e := range g[v] {			w := e.to			if newD := dist[v] + int64(e.wt); newD < dist[w] {				dist[w] = newD				q.push(pair86{w, newD})			}		}	}	for _, v := range leaf[1:] {		if dist[v] < inf {			Fprint(out, dist[v], " ")		} else {			Fprint(out, "-1 ")		}	}} //func main() { CF786B(os.Stdin, os.Stdout) }

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