7.18

                                    2017  D题 火车问题


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


 
源代码链接：点击打开链接

题目大意：给你一个数N，有1~N节按顺序排列的车厢
问你能否经过中转站C得到所指定的序列

思路： 中转站相当于一个栈，而达到指定序列有两种抢矿(1)一节一节出，即按原顺序到达B
                                                (2)先全部进栈，在全部出栈，即按逆序输出
             所以主要逻辑就是：

int a,b;
a=b=1;
 while(b<=n)
            {

                if(a==target[b])//(1)进去一个出来一个，即按顺序出站
                {
                    a++;
                    b++;
                }
                //(2)
                else if (!s.empty() && s.top()== target[b])//(2)和(3)判断逆序出站，并且如果中途顺序打乱转到(4)
                {
                    s.pop();
                    b++;
                }
                else if (a<=n)//(4)判断不满足条件的情况，即顺序打乱//逆序的话先进行压站把元素全部压完后再判断(2)操作是够完全逆序
                {
                    s.push(a);
                    a++;
                }
                else
                {
                    result=false;
                    break;
                }
            }