HDU 4514 湫湫系列故事——設計風景線(並查集+樹的直徑)

題目鏈接：
HDU 4514 湫湫系列故事——設計風景線
題意：
n 個點和m 條無向邊。先判環，無環的話輸出最長直徑。
數據範圍：n≤105,m≤106
分析：
並查集判環，樹的直徑兩遍dfs 即可。
用C++交AC,G++就MLE，o(╯□╰)o

//#pragma comment(linker,"/STACK:102400000, 102400000")
#include <stdio.h>
#include <string.h>
#include <algorithm>
#include <math.h>
#include <vector>
using namespace std;
typedef long long ll;
const int MAX_N = 100010;
const int MAX_M = 1000010;

int n, m;
int fa[MAX_N], dis[MAX_N], id[MAX_N], vis[MAX_N];
vector <pair<int, int> > vec[MAX_N];

int find(int x)
{
    return fa[x] == x ? x : fa[x] = find(fa[x]);
}

void dfs1(int u, int p)
{
    vis[u] = 1;
    for (int i = 0; i < vec[u].size(); ++i) {
        int v = vec[u][i].first, w = vec[u][i].second;
        if (v == p) continue;
        dfs1(v, u);
        if (dis[v] + w > dis[u]) {
            dis[u] = dis[v] + w;
            if (id[v] == 0) id[u] = v;
            else id[u] = id[v];
        } 
    }
}

void dfs2(int u, int p)
{
    vis[u] = 1;
    for (int i = 0; i < vec[u].size(); ++i) {
        int v = vec[u][i].first, w = vec[u][i].second;
        if (v == p) continue;
        dfs2(v, u);
        dis[u] = max(dis[u], dis[v] + w);
    }
}

void wyr()
{
    memset(fa, 0, sizeof(fa));
    memset(dis, 0, sizeof(dis));
    memset(vis, 0, sizeof(vis));
    memset(id, 0, sizeof(id));
    int ans = 0;
    for (int i = 1; i <= n; ++i) {
        if (vis[i] == 0) {
            dfs1(i, -1);
            memset(dis, 0, sizeof(dis));
            dfs2(id[i], -1);
            ans = max(ans, dis[id[i]]);
        }
    }
    printf("%d\n", ans);
}

int main()
{
    while (~scanf("%d%d", &n, &m)) {
        for (int i = 0; i <= n; ++i) {
            fa[i] = i;
            vec[i].clear();
        }
        int flag = 0;
        for (int i = 0; i < m; ++i) {
            int u, v, w;
            scanf("%d%d%d", &u, &v, &w);
            vec[u].push_back(make_pair(v, w));
            vec[v].push_back(make_pair(u, w));
            int fu = find(u), fv = find(v);
            if (fu == fv) flag = 1;
            else fa[fu] = fv;
        }
        if (flag) {
            printf("YES\n");
            continue;
        }
        wyr();
    }
    return 0;
}