[DFS]JZOJ 5916 flow

#include <iostream>
#include <cstdio>
using namespace std;
typedef long long ll;
const int N=2e5+10;
struct Edge {
    int u,v,nx;
}g[N*3];
int cnt,list[N];
ll w[N],f[N+N/2],sum;
bool vis[N];
int n,m;

void Add(int u,int v) {
    g[++cnt].u=u;g[cnt].v=v;g[cnt].nx=list[u];list[u]=cnt;
}

void Dfs(int u) {
    vis[u]=1;
    for (int i=list[u];i;i=g[i].nx)
        if (!vis[g[i].v]) {
            Dfs(g[i].v);
            w[u]+=w[g[i].v];
            f[i+1>>1]=(i&1?-1:1)*w[g[i].v];
        }
}

int main() {
    freopen("flow.in","r",stdin);
    freopen("flow.out","w",stdout);
    scanf("%d",&n);
    for (int i=1;i<=n;i++) scanf("%lld",&w[i]),w[i]=-w[i],sum+=w[i];
    if (sum) {
        printf("Impossible\n");
        return 0;
    }
    scanf("%d",&m);
    for (int i=1;i<=m;i++) {
        int u,v;
        scanf("%d%d",&u,&v);
        Add(u,v);Add(v,u);
    }
    Dfs(1);
    printf("Possible\n");
    for (int i=1;i<=m;i++) printf("%lld\n",f[i]);
}