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Problem solution · Go

Codeforces 605D — Board Game

Codeforces 605D — Board Game: a Go solution using sorting and greedy selection. Learn the idea, check the complexity, and read the full code, with credit to EndlessCheng Codeforces Go.

Technique
Sorting and greedy selection
Source
EndlessCheng Codeforces Go
Length
109 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

Sorting and greedy selection

For Codeforces 605D — Board Game, the implementation first exposes a useful order, then scans that order while making locally justified choices.

  1. Choose the key that reveals the greedy or grouping structure.
  2. Sort the relevant records by that key.
  3. Scan in order, maintaining the invariant that makes each local choice safe.

Code notes

  • 109 lines of Go from the credited upstream file 605D.go.
  • The implementation visibly relies on sequence storage.
  • No explicit loop blocks detected.

Complexity

Sorting is typically the dominant term unless the subsequent scan uses a more expensive nested operation.

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 605D — Board Game · GoGo
Use this to learn the idea, then write your own version.
package main import (	"bufio"	. "fmt"	"io"	"math/bits"	"slices"	"sort") // https://github.com/EndlessChengtype pair5 struct{ b, i int }var g5 [][]pair5 type seg5 []struct{ l, r, min int } func (t seg5) maintain(o int) {	t[o].min = min(t[o<<1].min, t[o<<1|1].min)} func (t seg5) build(o, l, r int) {	t[o].l, t[o].r = l, r	if l == r {		t[o].min = g5[l][0].b		return	}	m := (l + r) >> 1	t.build(o<<1, l, m)	t.build(o<<1|1, m+1, r)	t.maintain(o)} func (t seg5) delete(o, qr, maxY int, f func(int)) {	l := t[o].l	if l >= qr || t[o].min > maxY {		return	}	if l == t[o].r {		f(l)		t[o].min = g5[l][0].b		return	}	t.delete(o<<1|1, qr, maxY, f)	t.delete(o<<1, qr, maxY, f)	t.maintain(o)} func cf605D(in io.Reader, _w io.Writer) {	out := bufio.NewWriter(_w)	defer out.Flush()	var n int	Fscan(in, &n)	a := make([]struct{ a, b, c, d int }, n+1)	xs := make([]int, n+1)	for i := 1; i <= n; i++ {		Fscan(in, &a[i].a, &a[i].b, &a[i].c, &a[i].d)		xs[i] = a[i].a	}	slices.Sort(xs)	xs = slices.Compact(xs)	m := len(xs) 	g5 = make([][]pair5, m)	for i := 1; i <= n; i++ {		x := sort.SearchInts(xs, a[i].a)		g5[x] = append(g5[x], pair5{a[i].b, i})	}	for i, ps := range g5 {		slices.SortFunc(ps, func(a, b pair5) int { return a.b - b.b })		g5[i] = append(ps, pair5{2e9, 0}) // 哨兵	}	t := make(seg5, 2<<bits.Len(uint(m-1)))	t.build(1, 0, m-1) 	q := []int{0}	pre := make([]int, n+1)	for len(q) > 0 {		i := q[0]		q = q[1:]		if i == n {			ans := []int{}			for ; i > 0; i = pre[i] {				ans = append(ans, i)			}			Fprintln(out, len(ans))			for i := len(ans) - 1; i >= 0; i-- {				Fprint(out, ans[i], " ")			}			Fprintln(out)			return		} 		maxY := a[i].d		t.delete(1, sort.SearchInts(xs, a[i].c+1), maxY, func(l int) {			ps := g5[l]			for ps[0].b <= maxY {				pre[ps[0].i] = i				q = append(q, ps[0].i)				ps = ps[1:]			}			g5[l] = ps		})	}	Fprint(out, -1)} //func main() { cf605D(bufio.NewReader(os.Stdin), os.Stdout) }

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