Leha and another game about graph CodeForces - 840B (dfs)
阿新 • • 發佈:2019-02-12
get game force ces 給定 light void back problems
鏈接
大意: 給定無向連通圖, 每個點有權值$d_i$($-1\leq d_i \leq 1$), 求選擇一個邊的集合, 使得刪除邊集外的所有邊後, $d_i$不為-1的點的度數模2等於權值
首先要註意到該題只需要考慮dfs樹即可, 因為反向邊一定不會產生貢獻
存在權值為-1的點, 則直接以權值為-1的點為根dfs
若無權值為-1的點, 則答案不一定存在, 任選一個點為根dfs即可
#include <iostream> #include <algorithm> #include <math.h> #include <cstdio> #include <vector> #define REP(i,a,n) for(int i=a;i<=n;++i) #define x first #define y second #define pb push_back using namespace std; typedef pair<int,int> pii; const int N = 4e5+10, INF = 0x3f3f3f3f; int a[N], b[N], c[N], vis[N], f[N], n, m, k, t; vector<pii> g[N]; void dfs(int x) { if (vis[x]) return; vis[x]=1; for (auto e:g[x]) { dfs(e.x); if (a[e.x]==1) a[e.x]=0,f[e.y]^=1,a[x]^=1; } } int main() { scanf("%d%d", &n, &m); int rt = 1; REP(i,1,n) scanf("%d", a+i),a[i]==-1?rt=i:0; REP(i,1,m) { int u, v; scanf("%d%d",&u,&v); g[u].pb({v,i}),g[v].pb({u,i}); } dfs(rt); if (a[rt]==1) return puts("-1"),0; int cnt = 0; REP(i,1,m) cnt += f[i]; printf("%d\n", cnt); REP(i,1,m) if (f[i]) printf("%d ",i); puts(""); }
Leha and another game about graph CodeForces - 840B (dfs)