P2052 [NOI2011] road construction - a tree structure (water issues, chiefs do not enter)

P2052 [NOI2011] road construction

 

In fact, this problem can put things inside dfs finish work, (I also began to seek out a handful) ......

The contribution of one side is the son of the size and n-siz [v] Take the upper right;

#include<cmath>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int maxn=1e6+10;
typedef long long ll;
int pre[maxn*2],last[maxn],other[maxn*2],len[maxn*2],l;
void add(int x,int y,int z)
{
    l++;
    pre[l]=last[x];
    last[x]=l;
    other[l]=y;
    len[l]=z;
}

int n;
int father[maxn];
int siz[maxn];
ll ans;

void dfs(int x,int fa)
{
    siz[x]=1;
    for(int p=last[x];p;p=pre[p])
    {
        int v=other[p];
        if(v==fa) continue;
        dfs(v,x);
        ans+=(ll)abs(n-2*siz[v])*len[p];
        siz[x]+=siz[v];
    }
}



int main()
{
    scanf("%d",&n);
    for(int i=1;i<n;i++)
    {
        int a,b,c;
        scanf("%d%d%d",&a,&b,&c);
        add(a,b,c);
        add(b,a,c);
    }
    dfs(1,1);
    printf("%lld",ans); 
    return 0;
}

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