题目链接:http://acm.hdu.edu.cn/showproblem.php?pid=4514
题解:
先用一次dfs判断是否存在环,然后再枚举树的叶子节点找最长路。
#include<iostream>
#include<stdio.h>
#include<algorithm>
#include<cmath>
#include<stdlib.h>
#include <string.h>
#include<queue>
#include<set>
#include<map>
#include<stack>
#include<time.h>
#include<sstream>
using namespace std;
#define MAX_N 100005
#define inf 0x7f7f7f7f
#define LL long long
#define ull unsigned long long
const LL INF = 1e18;
const double Pi = acos(-1);
const int mod = 1e4+7;
typedef pair<int, int>P;
vector<P>G[MAX_N];
bool vis[MAX_N];
int n, m;
int mx;
int s;
void init()
{
memset(vis, false, sizeof(vis));
}
bool dfs(int u, int pre, int sum)
{
if(sum > mx) {
mx = sum;
s = u;
}
for(int i=0; i<G[u].size(); i++) {
int v = G[u][i].first;
int c = G[u][i].second;
if(v == pre)
continue;
if(vis[v])
return true;
vis[v] = true;
if(dfs(v, u, sum+c))
return true;
vis[v] = false;
}
return false;
}
int main()
{
while(~scanf("%d%d", &n, &m)) {
init();
for(int i=1; i<=n; i++)
G[i].clear();
for(int i=0; i<m; i++) {
int a, b, c;
scanf("%d%d%d", &a, &b, &c);
G[a].push_back(P(b, c));
G[b].push_back(P(a, c));
}
mx = 0;
vis[1] = true;
if(dfs(1, 0, 0)) {
puts("YES");
continue;
}
int ans = 0;
init();
for(int i=1; i<=n; i++) {
mx = 0;
if(G[i].size() > 1)//如果不是叶子节点
continue;
init();
vis[i] = true;
dfs(i, 0, 0);
ans = max(ans, mx);
}
cout << ans << endl;
}
}
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