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Problem solution · Go

Codeforces 821D — Okabe and City

Codeforces 821D — Okabe and City: 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
113 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 821D — Okabe and City, 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

  • 113 lines of Go from the credited upstream file 821D.go.
  • The implementation visibly relies on sequence storage, work queue.
  • 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 821D — Okabe and City · GoGo
Use this to learn the idea, then write your own version.
package main import (	"bufio"	. "fmt"	"io") // github.com/EndlessCheng/codeforces-gotype deque21 struct{ l, r []int } func (q deque21) empty() bool  { return len(q.l) == 0 && len(q.r) == 0 }func (q *deque21) pushL(v int) { q.l = append(q.l, v) }func (q *deque21) pushR(v int) { q.r = append(q.r, v) }func (q *deque21) popL() (v int) {	if len(q.l) > 0 {		q.l, v = q.l[:len(q.l)-1], q.l[len(q.l)-1]	} else {		v, q.r = q.r[0], q.r[1:]	}	return} func CF821D(_r io.Reader, out io.Writer) {	in := bufio.NewReader(_r)	min := func(a, b int) int {		if a < b {			return a		}		return b	} 	const inf int = 1e9	type pair struct{ x, y int }	var n, m, k, x, y int	Fscan(in, &n, &m, &k)	id := map[pair]int{}	for i := 0; i < k; i++ {		Fscan(in, &x, &y)		id[pair{x - 1, y - 1}] = i	} 	type nb struct{ to, wt int }	g := make([][]nb, k+n+m)	add := func(v, w int) {		g[v] = append(g[v], nb{w, 1})		g[w] = append(g[w], nb{v, 0})	}	for p, i := range id {		x, y := p.x, p.y		if x > 0 {			if j, has := id[pair{x - 1, y}]; has {				g[i] = append(g[i], nb{j, 0})				g[j] = append(g[j], nb{i, 0})			}		}		if y > 0 {			if j, has := id[pair{x, y - 1}]; has {				g[i] = append(g[i], nb{j, 0})				g[j] = append(g[j], nb{i, 0})			}		}		add(i, k+x)		if x > 0 {			add(i, k+x-1)		}		if x+1 < n {			add(i, k+x+1)		}		add(i, k+n+y)		if y > 0 {			add(i, k+n+y-1)		}		if y+1 < m {			add(i, k+n+y+1)		}	} 	st := id[pair{}]	dis := make([]int, len(g))	for i := range dis {		dis[i] = inf	}	dis[st] = 0	q := &deque21{}	q.pushL(st)	for !q.empty() {		v := q.popL()		for _, e := range g[v] {			w, d := e.to, e.wt			if newD := dis[v] + d; newD < dis[w] {				dis[w] = newD				if d == 0 {					q.pushL(w)				} else {					q.pushR(w)				}			}		}	}	ans := inf	if v, has := id[pair{n - 1, m - 1}]; has {		ans = dis[v]	}	ans = min(ans, min(dis[k+n-1], dis[k+n+m-1]))	if ans == inf {		ans = -1	}	Fprint(out, ans)} //func main() { CF821D(os.Stdin, os.Stdout) }

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