The diameter of the codeforces 14d two paths tree

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Give a tree

Find two paths with no duplicates to maximize the length product of the two paths.

Ideas:

1. To ensure that vertices are not repeated, delete an edge in the graph and enumerate the deleted edge.

2. In this way, we get two trees, and find the longest chain in their respective trees, that is, the diameter of the tree, and then multiply them.

#include<stdio.h>#include<iostream>#include<string.h>#include<set>#include<vector>#include<map>#include<math.h>#include<queue>#include<string>#include<stdlib.h>#include<algorithm>using namespace std;#define N 220struct Edge {int from, to, nex;bool hehe;}edge[N<<1];int head[N], edgenum;void add(int u, int v){Edge E={u,v,head[u],true};edge[edgenum] = E;head[u] = edgenum ++;}int n;int dis[N];void init(){ memset(head, -1, sizeof head);edgenum = 0; }int bfs(int x){memset(dis, 0, sizeof dis);dis[x] = 1;queue<int>q; q.push(x);int hehe = x;while(!q.empty()) {int u = q.front(); q.pop();for(int i = head[u]; ~i; i = edge[i].nex) {if(!edge[i].hehe)continue;int v = edge[i].to;if(dis[v])continue;dis[v] = dis[u]+1, q.push(v);if(dis[hehe]<dis[v])hehe = v;}}return hehe;}int main(){int i, u, v;while(~scanf("%d",&n)){init();for(i=1;i<n;i++){scanf("%d %d",&u,&v);add(u,v); add(v,u);}int ans = 0;for(i = 0; i < edgenum; i+=2){edge[i].hehe = edge[i^1].hehe = false;u = edge[i].from; v = edge[i].to;int L1 = bfs(u); int R1 = bfs(L1);int mul1 = dis[R1] -1;int L2 = bfs(v); int R2 = bfs(L2);int mul2 = dis[R2] -1;ans = max(ans, mul1 * mul2);edge[i].hehe = edge[i^1].hehe = true;}printf("%d\n",ans);}return 0;}