sicily 1024. Magic Island

Constraints

Time Limit: 1 secs, Memory Limit: 32 MB

Description

There are N cities and N-1 roads in Magic-Island. You can go from one city to any other. One road only connects two cities. One day, The king of magic-island want to visit the island from the capital. No road is visited twice. Do you know the longest distance the king can go.

Input

There are several test cases in the input
A test case starts with two numbers N and K. (1<=N<=10000, 1<=K<=N). The cities is denoted from 1 to N. K is the capital.

The next N-1 lines each contain three numbers X, Y, D, meaning that there is a road between city-X and city-Y and the distance of the road is D. D is a positive integer which is not bigger than 1000.
Input will be ended by the end of file.

Output

One number per line for each test case, the longest distance the king can go.

Sample Input

3 1
1 2 10
1 3 20

Sample Output

20

以前做的题，加了点注释

#include<cstdio>
#include<vector>
using namespace std;
#define MAX 10001
struct road {
    int id;  // id of the road
    int end; // another end
    int len;
    road(int i, int e, int l) {
        id = i, end = e; len = l;
    }
};

bool visited[MAX];  // has road id been visited
                    // can't use attr since the road is bidirenctional,
                    // whereas visiting is one-direnctional => unnecessary search
int maxLen;
vector<road> roads[MAX];

void dfs(int start, int total = 0) {
    // for each road connected to start
    for (int i = 0; i < roads[start].size(); i++) {
        // if it hasn't been visited
        if (!visited[roads[start][i].id]) {
            // visit it
            visited[roads[start][i].id] = true;
            total += roads[start][i].len;
             
            // upadate
            if (maxLen < total) 
                maxLen = total;
 
             // start from its other end
            dfs(roads[start][i].end, total);
            
            // [IMPORTANT] backtracking
            total -= roads[start][i].len;
            visited[roads[start][i].id] = false;
        }
    }
}

int main() {
    int start, end, len;
    int n, k;
    while(scanf("%d%d", &n, &k) != EOF) {
        // initialize
        for (int i = 0; i < MAX; i++) {
            roads[i].clear();
            visited[i] = false;
        }

        for (int i = 1; i < n; i++) {
            scanf("%d%d%d", &start, &end, &len);
            // bidirenctional
            roads[start].push_back(road(i, end, len));
            roads[end].push_back(road(i, start, len));
        }
        
        maxLen = 0;
        dfs(k);
        printf("%d\n", maxLen);
    }

    return 0;
}