sicily1024 Magic Island（图的遍历）

城市i对应的连接的路存到vector类型的cities[i]中；

每一条路有对应的id；

用visited[id]来记录是否已经走过这条路；

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

struct road{
    int id;            // every road has a unique id 
    int end;        // connect to which city
    int length;        // the length of this road
    road(int i, int e, int l) {
        id = i, end = e, length = l;
    }
};

#define MAX 10001
bool visited[MAX];    // when the road i is visited, visited[i] = true 
vector<road> cities[MAX];    // cities[i] stores all roads connecting the city i
int maxLength;

void dfs(int k, int total = 0) {
    for (int i = 0; i < cities[k].size(); i++) {
        // when the road has not visited
        if (!visited[cities[k][i].id]) {
            // visit the road
            visited[cities[k][i].id] = true;
            total += cities[k][i].length;
            if (total > maxLength) maxLength = total;
            // visit all roads connecting the city ‘cities[k][i].end‘
            dfs(cities[k][i].end, total);
            // unvisit the road
            visited[cities[k][i].id] = false;
            total -= cities[k][i].length;
        }
    }
}

int main() {
    int n, k;
    while (cin>>n>>k) {
        for (int i = 1; i <= n; i++) {    // initial
            visited[i] = false;
            cities[i].clear();
        }
        for (int i = 1; i < n; i++) {
            int x, y, l;
            cin>>x>>y>>l;
            cities[x].push_back(road(i, y, l));
            cities[y].push_back(road(i, x, l));
        }
        maxLength = 0;
        dfs(k);
        cout<<maxLength<<endl;
    }
    return 0;
}