Blue Bridge Cup - Minister of travel (BFS + diameter of the tree)

topic:

input Output:

Conclusion on the diameter of the tree:

Principle seeking tree diameter: beginning at any point x, do first BFS (DFS), find the furthest point y, then y this point, once BFS (DFS), find the furthest point z, z to y is in the tree diameter, denoted D (Z, Y) .

Code:

#include <iostream>
#include <cstring>
#include <cstdio>
#include <queue>
#include <vector>
using namespace std;

const int MAXN = 50005;   //刚开始开的数据太小 导致最后一个点过不去 
int dis[MAXN],head[MAXN],vis[MAXN];
int n,ind,ans,sourDot;
queue<int> q;

struct node
{
	int y,cost,next;
}edge[MAXN*2];

void Add(int x,int y,int len)
{
	edge[++ind].y = y;
	edge[ind].cost = len;
	edge[ind].next = head[x];
	head[x] = ind;
}

void BFS(int x)
{
	vis[x] = true;
	dis[x] = 0;
	q.push(x);
	while(!q.empty())
	{
		int tmpNo = q.front();
		q.pop();
		for(int i=head[tmpNo];i;i=edge[i].next)
		{
			if(!vis[edge[i].y])
			{
				vis[edge[i].y] = true;
				dis[edge[i].y] = dis[tmpNo] + edge[i].cost;
				if(dis[edge[i].y] > ans)
				{
					ans = dis[edge[i].y];
					sourDot = edge[i].y;
				}
				q.push(edge[i].y);
			}
		}
	}
}

int main()
{
	scanf("%d",&n);
	int x,y,len;
	for(int i=1;i<=n-1;++i)
	{
		scanf("%d%d%d",&x,&y,&len);
		Add(x,y,len);
		Add(y,x,len);
	}
	BFS(1);
	memset(vis,false,sizeof(vis));
	memset(dis,0,sizeof(dis));
	ans = 0;
	int tmp = sourDot;
	BFS(tmp);
	int res;
	if(ans%2)
		res = (ans+1)*(ans/2) + (ans+1)/2 + ans*10;
	else
	    res = (ans+1)*(ans/2) + ans*10;
	printf("%d\n",res);
	return 0;
}