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Problem solution · Go

Codeforces 999E — Reachability from the Capital

Codeforces 999E — Reachability from the Capital: a Go solution using depth-first search. Learn the idea, check the complexity, and read the full code, with credit to EndlessCheng Codeforces Go.

Technique
Depth-first search
Source
EndlessCheng Codeforces Go
Length
120 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

Depth-first search

For Codeforces 999E — Reachability from the Capital, the implementation follows one branch at a time, making it suitable for components, trees, backtracking, or dependency exploration.

  1. Define the state carried into one recursive or stack frame.
  2. Mark or choose the current state before exploring children.
  3. Combine child results or undo the choice when the branch finishes.

Code notes

  • 120 lines of Go from the credited upstream file 999E.go.
  • The implementation visibly relies on sequence storage.
  • No explicit loop blocks detected, together with recursive traversal.

Complexity

Count unique states for graph traversal; for backtracking, count the branching factor and maximum depth.

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 999E — Reachability from the Capital · GoGo
Use this to learn the idea, then write your own version.
package main import (	"bufio"	. "fmt"	"io") // github.com/EndlessCheng/codeforces-gofunc CF999E(_r io.Reader, _w io.Writer) {	in := bufio.NewReader(_r)	out := bufio.NewWriter(_w)	defer out.Flush() 	var n, m, st int	Fscan(in, &n, &m, &st)	st--	g := make([][]int, n)	type edge struct{ v, w int }	_edges := make([]edge, m)	for i := range _edges {		var v, w int		Fscan(in, &v, &w)		v--		w--		_edges[i] = edge{v, w}		g[v] = append(g[v], w)	} 	vis := make([]bool, n)	var f func(v int)	f = func(v int) {		vis[v] = true		for _, w := range g[v] {			if !vis[w] {				f(w)			}		}	}	f(st)	edges := []edge{}	for _, e := range _edges {		if !vis[e.v] && !vis[e.w] {			edges = append(edges, e)		}	} 	g = make([][]int, n)	rg := make([][]int, n)	for _, e := range edges {		v, w := e.v, e.w		g[v] = append(g[v], w)		rg[w] = append(rg[w], v)	}	used := make([]bool, n)	copy(used, vis)	vs := make([]int, 0, n)	var dfs func(int)	dfs = func(v int) {		used[v] = true		for _, w := range g[v] {			if !used[w] {				dfs(w)			}		}		vs = append(vs, v)	}	for v := range g {		if !used[v] {			dfs(v)		}	} 	used = make([]bool, n)	copy(used, vis)	var comp []int	var rdfs func(int)	rdfs = func(v int) {		used[v] = true		comp = append(comp, v)		for _, w := range rg[v] {			if !used[w] {				rdfs(w)			}		}	}	comps := [][]int{}	for i := len(vs) - 1; i >= 0; i-- {		if v := vs[i]; !used[v] {			comp = []int{}			rdfs(v)			comps = append(comps, comp)		}	} 	sccIDs := make([]int, n)	for i, cp := range comps {		for _, v := range cp {			sccIDs[v] = i		}	}	hasIn := make([]bool, len(comps))	for _, e := range edges {		if v, w := sccIDs[e.v], sccIDs[e.w]; v != w {			hasIn[w] = true		}	}	ans := 0	for _, has := range hasIn {		if !has {			ans++		}	}	Fprint(out, ans)} //func main() {//	CF999E(os.Stdin, os.Stdout)//}

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