hdu 1595 find the longest of the shortest （spfa）

#include<stdio.h>
#include<string.h>
#include<algorithm>
#include<queue>

using namespace std;

#define N 1005
const int inf = 1 << 28;

struct node{
    int to, nxt, w;
}e[N*N];

int dis
, vis
;
int head
;
int pre
;
int n, m;
int cnt;
int a

;

void init()
{
    memset(head, -1, sizeof(head));
    memset(pre, 0, sizeof(pre));
    memset(a, 0, sizeof(a));
    cnt = 0;
}

void add( int u, int v, int w )
{
    e[cnt].to = v;
    e[cnt].w = w;
    e[cnt].nxt = head[u];
    head[u] = cnt++;
}

void spfa()
{
	queue<int> q;
	while( !q.empty() )
		q.pop();
	for( int i = 1; i <= n; i++ )
	{
		vis[i] = 0;
		dis[i] = inf;
	}
	vis[1] = 1;
	dis[1] = 0;
	q.push(1);
	while( !q.empty() )
	{
		int now = q.front();
		q.pop();
		vis[now] = 0;
		for( int i = head[now]; ~i; i = e[i].nxt )
		{
			int to = e[i].to;
			if( dis[to] > dis[now] + e[i].w )
			{
				dis[to] = dis[now] + e[i].w;
				pre[to] = now;
				if( !vis[to] )
				{
					vis[to] = 1;
					q.push(to);
				}
			}
		}
	}
}

void spfa1()
{
	queue<int> q;
	while( !q.empty() )
		q.pop();
	for( int i = 1; i <= n; i++ )
	{
		vis[i] = 0;
		dis[i] = inf;
	}
	vis[1] = 1;
	dis[1] = 0;
	q.push(1);
	while( !q.empty() )
	{
		int now = q.front();
		q.pop();
		vis[now] = 0;
		for( int i = head[now]; ~i; i = e[i].nxt )
		{
			int to = e[i].to;
			if( a[now][to] && dis[to] > dis[now] + e[i].w )
			{
				dis[to] = dis[now] + e[i].w;
				//pre[to] = now;
				if( !vis[to] )
				{
					vis[to] = 1;
					q.push(to);
				}
			}
		}
	}
}

int main()
{
	while(~scanf("%d%d", &n, &m))
	{
		init();
		int u, v, w;
		while(m--)
		{
			scanf("%d%d%d", &u, &v, &w);
			add(u, v, w);
			add(v, u, w);
			a[u][v] = a[v][u] = 1;
		}
		int ans = 0;
		spfa();
		for( int i = n; i != 0; i = pre[i])
		{
			a[i][pre[i]] = a[pre[i]][i] = 0;
			spfa1();
			ans = max(ans, dis
);
			a[i][pre[i]] = a[pre[i]][i] = 1;
		}
		printf("%d\n", ans);
	}
	return 0;
}