Candies-POJ3159差分约束

Time Limit: 1500MS	Memory Limit: 131072K

Description

During the kindergarten days, flymouse was the monitor of his class. Occasionally the head-teacher brought the kids of flymouse’s class a large bag of candies and had flymouse distribute them. All the kids loved candies very much and often compared the numbers of candies they got with others. A kid A could had the idea that though it might be the case that another kid B was better than him in some aspect and therefore had a reason for deserving more candies than he did, he should never get a certain number of candies fewer than B did no matter how many candies he actually got, otherwise he would feel dissatisfied and go to the head-teacher to complain about flymouse’s biased distribution.

snoopy shared class with flymouse at that time. flymouse always compared the number of his candies with that of snoopy’s. He wanted to make the difference between the numbers as large as possible while keeping every kid satisfied. Now he had just got another bag of candies from the head-teacher, what was the largest difference he could make out of it?

Input

The input contains a single test cases. The test cases starts with a line with two integers N and M not exceeding 30 000 and 150 000 respectively. N is the number of kids in the class and the kids were numbered 1 through N. snoopy and flymouse were always numbered 1 and N. Then follow M lines each holding three integers A, B and c in order, meaning that kid A believed that kid B should never get over c candies more than he did.

Output

Output one line with only the largest difference desired. The difference is guaranteed to be finite.

Sample Input

2 2
1 2 5
2 1 4

Sample Output

5

Hint

32-bit signed integer type is capable of doing all arithmetic.

Source

POJ Monthly–2006.12.31, Sempr

题意：发一些糖果，其中他们之间的糖果数目有一定的约束关系，u,v,w 表示a[v]<=a[u]+w,求a[n]-a[1]的最大值

分析：典型的差分约束问题，不过在求最短路的过程中，不能用queue，会超时（不造什么原因），用stack。

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <string>
#include <queue>
#include <vector>
#include <stack>
#include <iostream>
#include <algorithm>

using namespace std;

const int MaxN = 31000;

const int MaxM = 151000;

const int INF = 0x3f3f3f3f;

typedef struct node
{
    int v,w,next;
}Line ;

Line Li[MaxM];

int Head[MaxN],top;

int Dis[MaxN];

bool vis[MaxN];

int n,m;

void AddEdge(int u,int v,int w)
{
    Li[top].v = v; Li[top].w = w;

    Li[top].next = Head[u];

    Head[u] = top++;
}

void SPFA()//求最短路
{
    stack<int>Q;//用stack

    memset(Dis,INF,sizeof(Dis));

    memset(vis,false,sizeof(vis));

    Dis[1] = 0 ;

    vis[1]=true;

    Q.push(1);

    while(!Q.empty())
    {
        int u = Q.top();

        Q.pop();

        for(int i = Head[u];i!=-1;i = Li[i].next)
        {
            int v = Li[i].v;

            if(Dis[v]>Dis[u]+Li[i].w)
            {
                Dis[v] = Dis[u]+Li[i].w;

                if(!vis[v])
                {
                    vis[v]=true;

                    Q.push(v);
                }
            }
        }

        vis[u] = false;
    }
}

int main()
{
    int u,v,w;

    while(~scanf("%d %d",&n,&m))
    {
        memset(Head,-1,sizeof(Head));

        top = 0;

        for(int i=0;i<m;i++)
        {
            scanf("%d %d %d",&u,&v,&w);

            AddEdge(u,v,w);
        }

        SPFA();

        printf("%d\n",Dis[n]-Dis[1]);
    }
    return 0;
}