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Problem solution · Go

Codeforces 1707C — DFS Trees

Codeforces 1707C — DFS Trees: 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
107 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 1707C — DFS Trees, 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

  • 107 lines of Go from the credited upstream file 1707C.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 1707C — DFS Trees · GoGo
Use this to learn the idea, then write your own version.
package main import (	"bufio"	"bytes"	. "fmt"	"io") // https://space.bilibili.com/206214func CF1707C(_r io.Reader, out io.Writer) {	in := bufio.NewReader(_r)	var n, m, v, w int	Fscan(in, &n, &m)	mst := make([][]int, n)	type edge struct{ x, y int }	nonMstEdges := []edge{}	fa := make([]int, n)	for i := range fa {		fa[i] = i	}	var find func(int) int	find = func(x int) int {		if fa[x] != x {			fa[x] = find(fa[x])		}		return fa[x]	}	for i := 0; i < m; i++ {		Fscan(in, &v, &w)		v--		w--		fv, fw := find(v), find(w)		if fv != fw {			mst[v] = append(mst[v], w)			mst[w] = append(mst[w], v)			fa[fv] = fw		} else {			nonMstEdges = append(nonMstEdges, edge{v, w})		}	} 	dep := make([]int, n)	var build func(int, int, int)	build = func(v, fa, d int) {		dep[v] = d		for _, w := range mst[v] {			if w != fa {				build(w, v, d+1)			}		}	}	build(0, -1, 0) 	check := make([][]int, n)	for _, e := range nonMstEdges {		v, w := e.x, e.y		if dep[v] < dep[w] { // 保证 w 的深度不超过 v			v, w = w, v		}		check[v] = append(check[v], w)	} 	diff := make([]int, n)	stk := make([]int, 0, n)	inStk := make([]bool, n)	var dfs func(int, int)	dfs = func(v, fa int) {		stk = append(stk, v)		inStk[v] = true		for _, w := range check[v] {			diff[v]++ // 子树 v 都是合法的			if inStk[w] { // w 是 v 的祖先				diff[0]++				diff[stk[dep[w]+1]]-- // 从 w 到 v 的路径上的中间节点（不含 v 和 w）都是不合法的			} else { // 横向边				diff[w]++ // 子树 w 都是合法的			}		}		for _, w := range mst[v] {			if w != fa {				dfs(w, v)			}		}		stk = stk[:len(stk)-1]		inStk[v] = false	}	dfs(0, -1) 	ans := bytes.Repeat([]byte{'0'}, n)	build = func(v, fa, s int) {		s += diff[v]		if s == m-n+1 {			ans[v] = '1'		}		for _, w := range mst[v] {			if w != fa {				build(w, v, s)			}		}	}	build(0, -1, 0)	Fprintf(out, "%s", ans)} //func main() { CF1707C(os.Stdin, os.Stdout) }

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