PAT(MOOC-数据结构) 06-4. How long does it take (25)

#include<cstring>
#include<iostream>
#include<fstream>
using namespace std;
//ifstream fin("06-4.txt");
//#define cin fin
#define MAX 101
typedef struct edge *ed;
struct edge{
int weight;
}edge[MAX][MAX];
int n, m;
int start, end1;
bool vst[MAX];//此题是因为要判定环路 才设定vst数组的 否则是不用的
int path = 0, max_path = -1;
bool dfs(int v){
if (path > max_path){
end1 = v;
max_path = path;
}
for (int i = 0; i < n; i++){
if (edge[v][i].weight > 0){
if (vst[i]){
return false;
}
path += edge[v][i].weight;
vst[i] = true;
if (!dfs(i))	return false;
path -= edge[v][i].weight;
vst[i] = false;
}
}
return true;
}
void dfs_re(int v){
if (path > max_path){
end1 = v;
max_path = path;
}
for (int i = 0; i < n; i++){
if (edge[i][v].weight > 0){
path += edge[i][v].weight;
dfs_re(i);
path -= edge[i][v].weight;
}
}
}
int main(){
cin >> n >> m;
memset(edge, -1, sizeof(edge));
int c1, c2, w;
for (int i = 0; i < m; i++){
cin >> c1 >> c2 >> w;
edge[c1][c2].weight = w;
}
int i, j;
for (j = 0; j < n; j++){
for (i = 0; i < n;i++)
if (edge[i][j].weight != -1)	break;
if (i == n)	break;
}
if (j == n){
printf("Impossible\n"); return 0;
}
start = j;
/*
从这个入度为0的点开始沿正向深度遍历，找到那最远的点
*/
vst[start] = true;
if (!dfs(start)){
printf("Impossible\n"); return 0;
}
vst[start] = false;
start = end1;/* 再从最远点逆序遍历 找最远点- - 只要求得max_path值即可 */
path = 0;
dfs_re(start);
printf("%d\n", max_path);
return 0;
}