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#include <bits/stdc++.h>
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using namespace std;
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const int N = 5e5 + 10, M = 4e6 + 10;
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//割边
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struct Node {
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int x, y;
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bool const operator<(const Node &t) const {
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if (x == t.x) return y < t.y;
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return x < t.x;
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}
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};
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vector<Node> v;
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//边双模板
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int e[M], h[N], idx, w[M], ne[M];
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void add(int a, int b, int c = 0) {
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e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
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}
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int dfn[N], low[N], stk[N], timestamp, top;
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int n, m;
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void tarjan(int u, int from) {
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dfn[u] = low[u] = ++timestamp;
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stk[++top] = u;
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for (int i = h[u]; ~i; i = ne[i]) {
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int j = e[i];
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if (!dfn[j]) {
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tarjan(j, i); // j:点，i:哪条边
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low[u] = min(low[u], low[j]);
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if (dfn[u] < low[j]) v.push_back({u, j});
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} else if (i != (from ^ 1))
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low[u] = min(low[u], dfn[j]);
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}
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}
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int main() {
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#ifndef ONLINE_JUDGE
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freopen("P1656.in", "r", stdin);
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#endif
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scanf("%d %d", &n, &m);
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memset(h, -1, sizeof h);
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while (m--) {
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int a, b;
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scanf("%d %d", &a, &b);
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if (a != b) add(a, b), add(b, a);
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}
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for (int i = 1; i <= n; i++)
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if (!dfn[i]) tarjan(i, -1);
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//要求先按a排序，然后按b排序
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sort(v.begin(), v.end());
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for (auto i : v) printf("%d %d\n", i.x, i.y);
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return 0;
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} |
