init

2D.py

@@ -0,0 +1,54 @@
+import sys
+
+data = sys.stdin.read().split()
+if not data:
+    sys.exit()
+n = int(data[0])
+m = int(data[1])
+adj = [[] for _ in range(n + 1)]
+idx = 2
+for i in range(1, m + 1):
+    u = int(data[idx])
+    v = int(data[idx+1])
+    idx += 2
+    adj[u].append((v, i))
+    adj[v].append((u, i))
+tin = [-1] * (n + 1)
+fup = [-1] * (n + 1)
+ans = []
+timer = 0
+ptr = [0] * (n + 1)
+stack = []
+for i in range(1, n + 1):
+    if tin[i] == -1:
+        tin[i] = fup[i] = timer
+        timer += 1
+        stack.append((i, -1))
+        while stack:
+            v, p_e = stack[-1]
+            if ptr[v] < len(adj[v]):
+                to, e_idx = adj[v][ptr[v]]
+                ptr[v] += 1
+                
+                if e_idx == p_e:
+                    continue
+                    
+                if tin[to] != -1:
+                    if tin[to] < fup[v]:
+                        fup[v] = tin[to]
+                else:
+                    tin[to] = fup[to] = timer
+                    timer += 1
+                    stack.append((to, e_idx))
+            else:
+                stack.pop()
+                if stack:
+                    p_v, _ = stack[-1]
+                    if fup[v] < fup[p_v]:
+                        fup[p_v] = fup[v]
+                    if fup[v] > tin[p_v]:
+                        ans.append(p_e)
+ans.sort()
+print(len(ans))
+if ans:
+    print(*(ans))