From 8d97766c3e6ff63fc0754d2819c3a8409ba0fdcf Mon Sep 17 00:00:00 2001
From: ia7ck <23146842+ia7ck@users.noreply.github.com>
Date: Sun, 23 Aug 2026 00:46:33 +0900
Subject: [PATCH 1/2] refactoring detect cycle undirected

---
 .../examples/cycle_detection_undirected.rs    |  49 +++++++++
 .../judge_cycle_detection_undirected.rs       | 102 ++++++++++++++++++
 libs/detect_cycle/src/lib.rs                  |  87 +++++++++------
 3 files changed, 205 insertions(+), 33 deletions(-)
 create mode 100644 libs/detect_cycle/examples/cycle_detection_undirected.rs
 create mode 100644 libs/detect_cycle/examples/judge_cycle_detection_undirected.rs

diff --git a/libs/detect_cycle/examples/cycle_detection_undirected.rs b/libs/detect_cycle/examples/cycle_detection_undirected.rs
new file mode 100644
index 00000000..6be16d4c
--- /dev/null
+++ b/libs/detect_cycle/examples/cycle_detection_undirected.rs
@@ -0,0 +1,49 @@
+// problem: https://judge.yosupo.jp/problem/cycle_detection_undirected
+// judge_program_rs: ./judge_cycle_detection_undirected.rs
+
+use detect_cycle::detect_cycle_undirected;
+use proconio::input;
+
+fn main() {
+    input! {
+        n: usize,
+        m: usize,
+        edges: [(usize, usize); m],
+    }
+    let Some(cycle) = detect_cycle_undirected(n, &edges) else {
+        println!("-1");
+        return;
+    };
+
+    let (u, v) = edges[cycle[0]];
+    let mut current = if cycle.len() == 1 {
+        u
+    } else {
+        let (next_u, next_v) = edges[cycle[1]];
+        if v == next_u || v == next_v { u } else { v }
+    };
+    let mut vertices = Vec::with_capacity(cycle.len());
+    for &edge_id in &cycle {
+        vertices.push(current);
+        let (u, v) = edges[edge_id];
+        current = if current == u { v } else { u };
+    }
+
+    println!("{}", cycle.len());
+    println!(
+        "{}",
+        vertices
+            .iter()
+            .map(|v| v.to_string())
+            .collect::<Vec<_>>()
+            .join(" ")
+    );
+    println!(
+        "{}",
+        cycle
+            .iter()
+            .map(|e| e.to_string())
+            .collect::<Vec<_>>()
+            .join(" ")
+    );
+}
diff --git a/libs/detect_cycle/examples/judge_cycle_detection_undirected.rs b/libs/detect_cycle/examples/judge_cycle_detection_undirected.rs
new file mode 100644
index 00000000..3b90f5ab
--- /dev/null
+++ b/libs/detect_cycle/examples/judge_cycle_detection_undirected.rs
@@ -0,0 +1,102 @@
+use proconio::{
+    input,
+    source::{Source, once::OnceSource},
+};
+
+use std::env;
+use std::fs::File;
+use std::io;
+use std::io::{BufRead, BufReader};
+
+struct Input {
+    n: usize,
+    m: usize,
+    edges: Vec<(usize, usize)>,
+}
+
+enum Output {
+    NotFound,
+    Found {
+        vertices: Vec<usize>,
+        edge_ids: Vec<usize>,
+    },
+}
+
+fn read_input(input: impl BufRead) -> Input {
+    let mut source = OnceSource::new(input);
+    input! {
+        from &mut source,
+        n: usize,
+        m: usize,
+        edges: [(usize, usize); m],
+    }
+    assert!(source.is_empty());
+    Input { n, m, edges }
+}
+
+fn read_output(output: impl BufRead) -> Output {
+    let mut source = OnceSource::new(output);
+    input! {
+        from &mut source,
+        len: isize,
+    }
+    if len < 0 {
+        assert_eq!(len, -1);
+        assert!(source.is_empty());
+        return Output::NotFound;
+    }
+
+    let len = len as usize;
+    assert!(len >= 1);
+    input! {
+        from &mut source,
+        vertices: [usize; len],
+        edge_ids: [usize; len],
+    }
+    assert!(source.is_empty());
+    Output::Found { vertices, edge_ids }
+}
+
+fn validate_cycle(input: &Input, vertices: &[usize], edge_ids: &[usize]) {
+    let len = vertices.len();
+    assert_eq!(edge_ids.len(), len);
+
+    let mut seen_vertices = vec![false; input.n];
+    let mut seen_edges = vec![false; input.m];
+    for i in 0..len {
+        let vertex = vertices[i];
+        let next_vertex = vertices[(i + 1) % len];
+        let edge_id = edge_ids[i];
+
+        assert!(vertex < input.n);
+        assert!(!seen_vertices[vertex]);
+        seen_vertices[vertex] = true;
+
+        assert!(edge_id < input.m);
+        assert!(!seen_edges[edge_id]);
+        seen_edges[edge_id] = true;
+
+        let (u, v) = input.edges[edge_id];
+        assert!((u == vertex && v == next_vertex) || (u == next_vertex && v == vertex));
+    }
+}
+
+fn main() -> io::Result<()> {
+    let args: Vec<String> = env::args().collect();
+    assert_eq!(args.len(), 4);
+    let (input, my_output, expected_output) = (&args[1], &args[2], &args[3]);
+
+    let input = read_input(BufReader::new(File::open(input)?));
+    let my_output = read_output(BufReader::new(File::open(my_output)?));
+    let expected_output = read_output(BufReader::new(File::open(expected_output)?));
+
+    match (my_output, expected_output) {
+        (Output::NotFound, Output::NotFound) => {}
+        (Output::Found { vertices, edge_ids }, Output::Found { .. }) => {
+            validate_cycle(&input, &vertices, &edge_ids)
+        }
+        _ => panic!("cycle existence does not match the expected output"),
+    }
+
+    Ok(())
+}
diff --git a/libs/detect_cycle/src/lib.rs b/libs/detect_cycle/src/lib.rs
index 4dcac88f..8b4a9e77 100644
--- a/libs/detect_cycle/src/lib.rs
+++ b/libs/detect_cycle/src/lib.rs
@@ -32,23 +32,26 @@
 pub fn detect_cycle_undirected(n: usize, edges: &[(usize, usize)]) -> Option<Vec<usize>> {
     fn dfs(
         curr: usize,
-        prev: usize,
+        prev_edge: Option<usize>,
         g: &[Vec<(usize, usize)>],
-        seen: &mut Vec<bool>,
-        parent: &mut Vec<(usize, usize)>,
-    ) -> Option<(usize, usize)> {
-        seen[curr] = true;
+        depth: &mut [Option<usize>],
+        path: &mut Vec<usize>,
+    ) -> Option<Vec<usize>> {
+        depth[curr] = Some(path.len());
         for &(nxt, idx) in &g[curr] {
-            if nxt == prev {
+            if Some(idx) == prev_edge {
                 continue;
             }
-            if seen[nxt] {
-                return Some((nxt, curr));
+            if let Some(nxt_depth) = depth[nxt] {
+                let mut cycle = path[nxt_depth..].to_vec();
+                cycle.push(idx);
+                return Some(cycle);
             }
-            parent[nxt] = (curr, idx);
-            if let Some((start, end)) = dfs(nxt, curr, g, seen, parent) {
-                return Some((start, end));
+            path.push(idx);
+            if let Some(cycle) = dfs(nxt, Some(idx), g, depth, path) {
+                return Some(cycle);
             }
+            path.pop();
         }
         None
     }
@@ -58,31 +61,15 @@ pub fn detect_cycle_undirected(n: usize, edges: &[(usize, usize)]) -> Option<Vec
         g[u].push((v, idx));
         g[v].push((u, idx));
     }
-    let mut seen = vec![false; n];
-    let mut parent = vec![(!0, !0); n];
+    let mut depth = vec![None; n];
+    let mut path = Vec::new();
 
     for v in 0..n {
-        if seen[v] {
+        if depth[v].is_some() {
             continue;
         }
-        if let Some((cycle_start, cycle_end)) = dfs(v, !0, &g, &mut seen, &mut parent) {
-            let mut cycle = Vec::new();
-            {
-                let mut curr = cycle_end;
-                while curr != cycle_start {
-                    let (par, idx) = parent[curr];
-                    cycle.push(idx);
-                    curr = par;
-                }
-            }
-            // cycle_end <- parent[cycle_end] <- parent[parent[cycle_end]] <- ... <- cycle_start
-            for (idx, &(u, v)) in edges.iter().enumerate() {
-                if (u, v) == (cycle_start, cycle_end) || (u, v) == (cycle_end, cycle_start) {
-                    cycle.push(idx);
-                    return Some(cycle);
-                }
-            }
-            unreachable!();
+        if let Some(cycle) = dfs(v, None, &g, &mut depth, &mut path) {
+            return Some(cycle);
         }
     }
     None
@@ -165,7 +152,41 @@ pub fn detect_cycle_directed(n: usize, edges: &[(usize, usize)]) -> Option<Vec<u
 
 #[cfg(test)]
 mod tests {
-    use crate::detect_cycle_directed;
+    use crate::{detect_cycle_directed, detect_cycle_undirected};
+
+    #[test]
+    fn test_undirected_triangle() {
+        let cycle = detect_cycle_undirected(3, &[(0, 2), (2, 1), (1, 0)]).unwrap();
+        assert!(
+            [
+                vec![0, 1, 2],
+                vec![1, 2, 0],
+                vec![2, 0, 1],
+                vec![0, 2, 1],
+                vec![2, 1, 0],
+                vec![1, 0, 2],
+            ]
+            .contains(&cycle)
+        );
+    }
+
+    #[test]
+    fn test_undirected_v() {
+        let cycle = detect_cycle_undirected(3, &[(0, 2), (0, 1)]);
+        assert_eq!(cycle, None);
+    }
+
+    #[test]
+    fn test_undirected_parallel_edges() {
+        let cycle = detect_cycle_undirected(2, &[(0, 1), (0, 1)]).unwrap();
+        assert!([vec![0, 1], vec![1, 0]].contains(&cycle));
+    }
+
+    #[test]
+    fn test_undirected_self_loop() {
+        let cycle = detect_cycle_undirected(1, &[(0, 0)]);
+        assert_eq!(cycle, Some(vec![0]));
+    }
 
     #[test]
     fn test_directed_triangle() {

From d5faf5d28aca441a9a01a1d37eec2cab38e15477 Mon Sep 17 00:00:00 2001
From: ia7ck <23146842+ia7ck@users.noreply.github.com>
Date: Sun, 23 Aug 2026 00:47:18 +0900
Subject: [PATCH 2/2] stack unlimited

---
 .github/workflows/ci.yml | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 38dedffe..7ad06424 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -72,7 +72,9 @@ jobs:
     - name: Release build
       run: cargo build --release --examples
     - name: oj test
-      run: cargo run --release --bin oj_test
+      run: |
+        ulimit -s unlimited
+        cargo run --release --bin oj_test
 
   publish-doc:
     runs-on: ubuntu-latest