U81904 【模板】树的直径

1. 两次Dfs

注意：该方法无法解决负权树。

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#include<cstdio>
#include<cstring>
#include<algorithm>
#define re register
using namespace std;
const int MAXn = 5e5;
const int MAXm = MAXn;
const int FINF = 0xc0c0c0c0;

int n;
int head[MAXn + 10], cntnex, nex[MAXm * 2 + 10], to[MAXm * 2 + 10], wei[MAXm * 2 + 10];
void Insert(int u, int v, int w) {
    nex[++cntnex] = head[u];
    head[u] = cntnex;
    to[cntnex] = v;
    wei[cntnex] = w;
}

int dis[MAXn + 10]; bool vis[MAXn + 10];
void Dfs(int cur) {
    vis[cur] = 1;
    for (re int i = head[cur]; i; i = nex[i]) {
        if (vis[to[i]]) continue;
        dis[to[i]] = dis[cur] + wei[i];
        Dfs(to[i]);
    }
}
int EvaFar(int root) {
    memset(dis, 0xc0, sizeof(dis));
    memset(vis, 0, sizeof(vis));
    dis[root] = 0;
    Dfs(root);
    int mx = FINF, maxer = 0;
    for (re int i = 1; i <= n; ++i) {
        if (mx < dis[i]) {
            mx = dis[i];
            maxer = i;
        }
    }
    return maxer;
}

int side1, side2;
int main() {
    scanf("%d", &n);
    for (re int i = 1, u, v, w; i < n; ++i) {
        scanf("%d %d %d", &u, &v, &w);
        Insert(u, v, w);
        Insert(v, u, w);
    }
    side1 = EvaFar(1);
    side2 = EvaFar(side1);
    printf("%d %d\n", side1, side2);
    printf("%d\n", dis[side2]);
}

2. 树形Dp

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#include<bits/stdc++.h>
using namespace std;
const int MAXn = 5e5;
const int MAXm = MAXn;

template <typename T>
inline void read(T &a) {
    char c;for (c = getchar(); (c < '0' || c > '9') && c != '-'; c = getchar());bool f = c == '-';T x = f ? 0 : (c ^ '0');for (c = getchar(); c >= '0' && c <= '9'; c = getchar()) {x = x * 10 + (c ^ '0');}a = f ? -x : x;
}
template <typename T, typename ...Argv>
inline void read(T &a, Argv &...argv) {
    read(a), read(argv...);
}

int head[MAXn + 10], cntnex, nex[MAXm * 2 + 10], to[MAXm * 2 + 10], wei[MAXm * 2 + 10];
inline void Insert(int u, int v, int w) {
    nex[++cntnex] = head[u];
    head[u] = cntnex;
    to[cntnex] = v;
    wei[cntnex] = w;
}

int ans, d[MAXn + 10];
void Dfs(int cur, int fath) {
    for (int i = head[cur]; i; i = nex[i]) {
        if (to[i] == fath) continue;
        Dfs(to[i], cur);
        ans = max(ans, d[cur] + d[to[i]] + wei[i]);
        d[cur] = max(d[cur], d[to[i]] + wei[i]);
    }
}

int n;
signed main() {
    read(n);
    for (int i = 1, u, v, w; i < n; ++i) {
        read(u, v, w);
        Insert(u, v, w); Insert(v, u, w);
    }
    Dfs(1, 0);
    printf("%d\n", ans);
    return 0;
}