【DFS——Codeforces Beta Round #14 (Div. 2)】D. Two Paths

#include <map>
#include <set>
#include <list>
#include <queue>
#include <deque>
#include <stack>
#include <string>
#include <time.h>
#include <cstdio>
#include <math.h>
#include <iomanip>
#include <cstdlib>
#include <limits.h>
#include <string.h>
#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;

#define LL long long
#define MIN INT_MIN
#define MAX INT_MAX
#define PI acos(-1.0)
#define FRE freopen ("input.txt","r",stdin)
#define FF  freopen ("output.txt","w",stdout)
#define N 205
vector<int> v
;
int maxm;
int dep;
int dfs(int x,int pre) {
int res = 0;
int d1 = 0,d2 = 0;//最大&次大
int i,j;
for (i = 0; i < v[x].size(); i++) {
if (v[x][i] != pre) {
res = max(res,dfs(v[x][i],x));
if (dep > d1) {
d2 = d1;
d1 = dep;
} else {
d2 = max(dep,d2);
}
}
}
res = max(res,d1+d2);
dep = d1 + 1;
return res;
}

int main () {
int n;
scanf("%d",&n);
int i,j;
int t = n - 1;
while  (t--) {
int a,b;
scanf("%d%d",&a,&b);
v[a].push_back(b);
v[b].push_back(a);
}
maxm = 0;
//枚举每条边
for (i = 1; i <= n; i++) {
for (j = 0; j < v[i].size(); j++) {
int x = i;
int y = v[i][j];
int a = dfs(x,y);
int b = dfs(y,x);
if(a*b > maxm) {
maxm = a*b;
}
}
}
printf("%d\n",maxm);
return 0;
}