Codeforces 405E Graph Cutting

Graph Cutting

不会写。。

dfs的过程中把回边丢到它的祖先中去， 回溯的时候两两配对。感觉好神奇啊。

#include<bits/stdc++.h>
#define LL long long
#define fi first
#define se second
#define mk make_pair
#define PLL pair<LL, LL>
#define PLI pair<LL, int>
#define PII pair<int, int>
#define SZ(x) ((int)x.size())
#define ull unsigned long long

using namespace std;

const int N = 1e5 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + 7;
const double eps = 1e-8;
const double PI = acos(-1);

int n, m;
bool vis[N];
vector<int> G[N];
queue<int> que[N];
vector<pair<int, PII>> ans;

void dfs(int u, int fa) {
    vis[u] = true;
    for(auto& v : G[u]) {
        if(!vis[v]) {
            dfs(v, u);
        } else if(v != fa && vis[v]) {
            que[v].push(u);
        }
    }
    while(SZ(que[u]) >= 2) {
        int x = que[u].front(); que[u].pop();
        int y = que[u].front(); que[u].pop();
        ans.push_back(mk(x, mk(u, y)));
    }
    if(SZ(que[u])) {
        if(!fa) {
            puts("No solution");
            exit(0);
        }
        int x = que[u].front(); que[u].pop();
        ans.push_back(mk(x, mk(u, fa)));
    } else {
        que[fa].push(u);
    }
}

int main() {
    scanf("%d%d", &n, &m);
    for(int i = 1; i <= m; i++) {
        int u, v; scanf("%d%d", &u, &v);
        G[u].push_back(v);
        G[v].push_back(u);
    }
    if(m & 1) puts("No solution");
    else {
        dfs(1, 0);
        for(auto& t : ans)
            printf("%d %d %d
", t.fi, t.se.fi, t.se.se);
    }
    return 0;
}

/*
*/