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Problem solution · Go

Codeforces 962F — Simple Cycles Edges

Codeforces 962F — Simple Cycles Edges: 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
100 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 962F — Simple Cycles Edges, 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

  • 100 lines of Go from the credited upstream file 962F.go.
  • The implementation visibly relies on sequence storage.
  • 1 loop block 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 962F — Simple Cycles Edges · GoGo
Use this to learn the idea, then write your own version.
package main import (	"bufio"	. "fmt"	"io") // github.com/EndlessCheng/codeforces-gofunc CF962F(_r io.Reader, _w io.Writer) {	in := bufio.NewReader(_r)	out := bufio.NewWriter(_w)	defer out.Flush()	min := func(a, b int) int {		if a < b {			return a		}		return b	} 	var n, m, v, w int	Fscan(in, &n, &m)	type nei struct{ to, eid int }	g := make([][]nei, n)	for i := 1; i <= m; i++ {		Fscan(in, &v, &w)		v--		w--		g[v] = append(g[v], nei{w, i})		g[w] = append(g[w], nei{v, i})	} 	onSimpleCycle := make([]bool, m+1)	bccIDs := make([]int, n)	idCnt := 0	dfn := make([]int, n)	dfsClock := 0	type edge struct{ v, w, eid int }	stack := []edge{}	var f func(v, fa int) int	f = func(v, fa int) int {		dfsClock++		dfn[v] = dfsClock		lowV := dfsClock		for _, ne := range g[v] {			w := ne.to			e := edge{v, w, ne.eid}			if dfn[w] == 0 {				stack = append(stack, e)				lowW := f(w, v)				if lowW >= dfn[v] {					idCnt++					var comp, eids []int					for {						e, stack = stack[len(stack)-1], stack[:len(stack)-1]						if bccIDs[e.v] != idCnt {							bccIDs[e.v] = idCnt							comp = append(comp, e.v)						}						if bccIDs[e.w] != idCnt {							bccIDs[e.w] = idCnt							comp = append(comp, e.w)						}						eids = append(eids, e.eid)						if e.v == v && e.w == w {							break						}					}					if len(comp) == len(eids) {						for _, eid := range eids {							onSimpleCycle[eid] = true						}					}				}				lowV = min(lowV, lowW)			} else if w != fa && dfn[w] < dfn[v] {				stack = append(stack, e)				lowV = min(lowV, dfn[w])			}		}		return lowV	}	for v, ts := range dfn {		if ts == 0 && len(g[v]) > 0 {			f(v, -1)		}	} 	ans := []interface{}{}	for i, b := range onSimpleCycle {		if b {			ans = append(ans, i)		}	}	Fprintln(out, len(ans))	Fprint(out, ans...)} //func main() { CF962F(os.Stdin, os.Stdout) }

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