POJ 3159

POJ 3159 Candies（差分规划+SPFA）CandiesTime Limit: 1500MS Memory Limit: 131072KTotal Submissions: 18737 Accepted: 4930DescriptionDuring 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?InputThe 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.OutputOutput one line with only the largest difference desired. The difference is guaranteed to be finite.Sample Input2 21 2 52 1 4Sample Output5//spfa+stack 求最短路径    //这里求最大相差糖果数，意思是在各位同学的意愿下，班长比班级的表现最差的同学相差的最大糖果树 //意为在所有情况下，班长比这位同学相差的最小糖果数 #include<stdio.h>#include<iostream>#include<algorithm>#include<string.h>using namespace std;const int MAXN=30010;const int MAXE=150010;const int INF=0x3f3f3f3f;int head[MAXN];//每个结点的头指针int vis[MAXN];//在队列标志int Q[MAXN];//堆栈int dist[MAXN];struct Edge{    int to;    int v;    int next;}edge[MAXE];//边数组 int tol;void add(int a,int b,int v)//加边//b比a大v {    edge[tol].to=b;//边的起点     edge[tol].v=v;//边的权值     edge[tol].next=head[a];//记录这条边的顶点的标号     head[a]=tol++;//tol的初始化为0 ，改变此顶点的标号 }void SPFA(int start,int n){    int top=0;    for(int v=1;v<=n;v++)//初始化    {        if(v==start)        {            Q[top++]=v;            vis[v]=true;            dist[v]=0;        }        else        {            vis[v]=false;            dist[v]=INF;        }    }    while(top!=0)    {        int u=Q[--top];        vis[u]=false;        for(int i=head[u];i!=-1;i=edge[i].next)        {            int v=edge[i].to;            if(dist[v]>dist[u]+edge[i].v)            {                dist[v]=dist[u]+edge[i].v;                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        if(!vis[v])                {                    vis[v]=true;                    Q[top++]=v;                }            }        }    }}int main(){   // freopen("in.txt","r",stdin);  //  freopen("out.txt","w",stdout);    int n;    int M;    int a,b,c;    while(scanf("%d%d",&n,&M)!=EOF)    {        tol=0;//加边计数，这个不要忘        memset(head,-1,sizeof(head));        while(M--)        {            scanf("%d%d%d",&a,&b,&c);            //b-a<=c            add(a,b,c);            //大-小<=c ,有向边(小，大）：c        }        SPFA(1,n);        printf("%d\n",dist[n]);    }    return 0;}注：  spfa+stack  spfa+queue  一个是bfs（广搜）加迭代  一个是dfs（深搜）加迭代  迭代：就是不断的做，做到没有更有的解为止  或者是不断的做，做到逼近答案为止  栈比队列更快更节省空间