51Nod 2602 树的直径

题目链接：51Nod 2602 树的直径

题目大意：

题解：
两遍\(dfs\)，第一遍从根开始找到最深的结点，第二遍从最深的结点开始得到树的直径。

#include <cstring>
#include <iostream>
using namespace std;

struct Edge {
    int v, next;
} edge[200010];
int head[100010], cnt;
int n, dis[100010], start;

void addEdge(int u, int v) {
    edge[++cnt].v = v;
    edge[cnt].next = head[u];
    head[u] = cnt;
}

void dfs(int x, int fa) {
    for (int i = head[x]; i; i = edge[i].next) {
        int v = edge[i].v;
        if (v != fa) {
            dis[v] = dis[x] + 1;
            dfs(v, x);
        }
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin >> n;
    for (int i = 1; i < n; ++i) {
        int x, y;
        cin >> x >> y;
        addEdge(x, y);
        addEdge(y, x);
    }
    memset(dis, 0x3f3f3f3f, sizeof(dis));
    dis[1] = 0;
    dfs(1, 0);
    int maxn = -1;
    for (int i = 1; i <= n; ++i) {
        if (dis[i] > maxn) {
            maxn = dis[i];
            start = i;  // 树的直径的起点
        }
    }
    memset(dis, 0x3f3f3f3f, sizeof(dis));
    dis[start] = 0;
    dfs(start, 0);
    maxn = -1;
    for (int i = 1; i <= n; ++i) {
        if (dis[i] > maxn) {
            maxn = dis[i];
        }
    }
    cout << maxn << endl;
    return 0;
}