#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, m;
cin >> n >> m;
vector<array<int, 2>> a(m), inv(n, {-1, -1}), ans;
auto drivable = [&](int i, int p) {
assert(i < n && p < 2);
int pos = inv[i][p];
if (pos == -1) {
while (true) {
cout << "axaxa" << endl;
}
}
return a[pos][0] == i || a[pos][0] == -1;
};
for (int i = 0; i < m; ++i) {
cin >> a[i][1] >> a[i][0];
--a[i][1], --a[i][0];
for (int k : a[i]) {
if (k != -1) {
if (inv[k][0] == -1) {
inv[k][0] = i;
} else {
inv[k][1] = i;
}
}
}
}
vector<bool> done(n);
vector<int> free;
for (int i = 0; i < m; ++i) {
if (a[i][0] == -1 && a[i][1] == -1) {
free.push_back(i);
}
}
for (int i = 0; i < n; ++i) {
if (inv[i][0] == inv[i][1]) {
done[i] = true;
}
}
while (find(done.begin(), done.end(), false) != done.end()) {
bool any = false;
for (int i = 0; i < n; ++i) {
if (!done[i]) {
int from = inv[i][0], to = inv[i][1];
if (drivable(i, 0) && drivable(i, 1) && (a[from][0] == -1 || a[to][0] == -1)) {
if (a[to][0] != -1) {
swap(from, to);
}
done[i] = true;
any = true;
ans.push_back({from, to});
a[from][(a[from][1] == i)] = -1;
a[to][0] = i;
inv[i][0] = inv[i][1] = to;
if (a[from][1] == -1) {
free.push_back(from);
}
}
}
}
if (!any) {
for (int i = 0; i < n; ++i) {
if (!done[i]) {
int x = inv[i][0], y = inv[i][1];
if (drivable(i, 0) && drivable(i, 1) && !free.empty()) {
ans.push_back({x, free.back()});
ans.push_back({y, free.back()});
inv[i][0] = inv[i][1] = free.back();
free.pop_back();
a[x][0] = -1, a[y][0] = -1;
done[i] = true;
any = true;
}
}
}
if (!any) {
break;
}
}
}
if (m > 4 && find(done.begin(), done.end(), false) != done.end()) {
assert(false);
if (free.empty()) {
cout << "-1\n";
return 0;
}
for (int i = 0; i < n; ++i) {
if (!done[i]) {
int &x = inv[i][0], &y = inv[i][1];
if (a[x][0] != i) {
swap(x, y);
}
assert(a[inv[i][0]][0] == i && a[inv[i][1]][1] == i);
}
}
for (int i = 0; i < n; ++i) {
if (!done[i]) {
int x = inv[i][0];
int y = inv[i][0];
vector<int> cycle;
do {
cycle.push_back(y);
y = inv[a[y][1]][0];
} while (x != y);
ans.push_back({cycle[0], free.back()});
for (int j = 1; j < size(cycle); ++j) {
ans.push_back({cycle[j], cycle[j - 1]});
}
ans.push_back({free.back(), cycle.back()});
for (int j : cycle) {
done[a[j][0]] = done[a[j][1]] = true;
}
}
}
}
cout << ans.size() << '\n';
for (auto [x, y] : ans) {
cout << x + 1 << " " << y + 1 << '\n';
}
return 0;
}
Compilation message
Main.cpp: In function 'int main()':
Main.cpp:135:35: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
135 | for (int j = 1; j < size(cycle); ++j) {
| ~~^~~~~~~~~~~~~
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
1 ms |
212 KB |
Output is correct |
| 2 |
Correct |
0 ms |
212 KB |
Output is correct |
| 3 |
Correct |
0 ms |
212 KB |
Output is correct |
| 4 |
Incorrect |
0 ms |
212 KB |
Output isn't correct |
| 5 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Incorrect |
55 ms |
4772 KB |
Output isn't correct |
| 2 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Runtime error |
1 ms |
468 KB |
Execution killed with signal 6 |
| 2 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Runtime error |
1 ms |
468 KB |
Execution killed with signal 6 |
| 2 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Incorrect |
1 ms |
340 KB |
Output isn't correct |
| 2 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
1 ms |
212 KB |
Output is correct |
| 2 |
Correct |
0 ms |
212 KB |
Output is correct |
| 3 |
Correct |
0 ms |
212 KB |
Output is correct |
| 4 |
Incorrect |
0 ms |
212 KB |
Output isn't correct |
| 5 |
Halted |
0 ms |
0 KB |
- |
