ACM/ICPC CERC 1997,UVa 514,POJ 1363 Rails(栈模拟)

原题

Rails

Time Limit:1000MS Memory Limit:10000KB 64bit IO Format:%I64d & %I64u

Description

There is a famous railway station in PopPush City. Country there is incredibly hilly. The station was built in last century. Unfortunately, funds were extremely limited that time. It was possible to establish only a surface track. Moreover, it turned out that the station could be only a dead-end one (see picture) and due to lack of available space it could have only one track.

The local tradition is that every train arriving from the direction A continues in the direction B with coaches reorganized in some way. Assume that the train arriving from the direction A has N <= 1000 coaches numbered in increasing order 1, 2, ..., N. The chief for train reorganizations must know whether it is possible to marshal coaches continuing in the direction B so that their order will be a1, a2, ..., aN. Help him and write a program that decides whether it is possible to get the required order of coaches. You can assume that single coaches can be disconnected from the train before they enter the station and that they can move themselves until they are on the track in the direction B. You can also suppose that at any time there can be located as many coaches as necessary in the station. But once a coach has entered the station it cannot return to the track in the direction A and also once it has left the station in the direction B it cannot return back to the station.

Input

The input consists of blocks of lines. Each block except the last describes one train and possibly more requirements for its reorganization. In the first line of the block there is the integer N described above. In each of the next lines of the block there is a permutation of 1, 2, ..., N. The last line of the block contains just 0.

The last block consists of just one line containing 0.

Output

The output contains the lines corresponding to the lines with permutations in the input. A line of the output contains Yes if it is possible to marshal the coaches in the order required on the corresponding line of the input. Otherwise it contains No. In addition, there is one empty line after the lines corresponding to one block of the input. There is no line in the output corresponding to the last ``null'' block of the input.

Sample Input

5
1 2 3 4 5
5 4 1 2 3
0
6
6 5 4 3 2 1
0
0

Sample Output

Yes
No

Yes

题意

如上图所示，已知火车车厢要以1~N的顺序从A入站，然后从station出站到B。问能不能通过站台改变车厢出站顺序来实现按所给顺序出站。

例如，出站顺序 2 1 3 4 5 是可能的：先让1 2进站，在让它们出站，然后让3 4 5，依次进站，出站。

出站顺序5 4 1 2 3是不可能的，因为5先出站意味着站中丛里到外为 1 2 3 4 5，只能以5 4 3 2 1顺序出站。

涉及知识及算法

把station看做是一个栈，按1~N的顺序，先入栈一个车厢，如果栈顶的车厢编号和所给出的编号一样。那么火车就出栈，循环直到栈里边所有满足出站顺序的火车都出站或栈空，否则就入栈。最后判断所有火车是否都出站了。若都出站，输出Yes，否则输出No。

代码

#include <cstdio>#include <stack>#include <iostream>using namespace std;const int MAXN =1000+10;int n,target[MAXN];int main(){    //freopen("in.txt","r",stdin);    while(scanf("%d",&n)&&n)    {        while(scanf("%d",&target[1])&&target[1])        {            stack<int> s;            int A=1,B=1;            for(int i=2;i<=n;i++)            {                scanf("%d",&target[i]);            }            int ok=1;            while(A<=n)            {                s.push(A);                while(!s.empty()&&s.top()==target[B])                {                    s.pop();                    B++;                }                A++;            }            if(!s.empty()) ok=0;            printf("%s\n",ok?"Yes":"No");        }        printf("\n");    }    return 0;}