【Codeforces 821C. Okabe and Boxes】

C. Okabe and Boxes
time limit per test3 seconds
memory limit per test256 megabytes
inputstandard input
outputstandard output
Okabe and Super Hacker Daru are stacking and removing boxes. There are n boxes numbered from 1 to n. Initially there are no boxes on the stack.

Okabe, being a control freak, gives Daru 2n commands: n of which are to add a box to the top of the stack, and n of which are to remove a box from the top of the stack and throw it in the trash. Okabe wants Daru to throw away the boxes in the order from 1 to n. Of course, this means that it might be impossible for Daru to perform some of Okabe’s remove commands, because the required box is not on the top of the stack.

That’s why Daru can decide to wait until Okabe looks away and then reorder the boxes in the stack in any way he wants. He can do it at any point of time between Okabe’s commands, but he can’t add or remove boxes while he does it.

Tell Daru the minimum number of times he needs to reorder the boxes so that he can successfully complete all of Okabe’s commands. It is guaranteed that every box is added before it is required to be removed.

Input
The first line of input contains the integer n (1 ≤ n ≤ 3·105) — the number of boxes.

Each of the next 2n lines of input starts with a string “add” or “remove”. If the line starts with the “add”, an integer x (1 ≤ x ≤ n) follows, indicating that Daru should add the box with number x to the top of the stack.

It is guaranteed that exactly n lines contain “add” operations, all the boxes added are distinct, and n lines contain “remove” operations. It is also guaranteed that a box is always added before it is required to be removed.

Output
Print the minimum number of times Daru needs to reorder the boxes to successfully complete all of Okabe’s commands.

Examples
input
3
add 1
remove
add 2
add 3
remove
remove
output
1
input
7
add 3
add 2
add 1
remove
add 4
remove
remove
remove
add 6
add 7
add 5
remove
remove
remove
output
2
Note
In the first sample, Daru should reorder the boxes after adding box 3 to the stack.

In the second sample, Daru should reorder the boxes after adding box 4 and box 7 to the stack.

题意 ： 两种操作
1） add 在栈顶加入一个元素
2） remove 拿出栈顶的元素
要求最后的出栈顺序为递增

思路 ： 数组模拟一个栈，若栈顶元素非最下的，排序
排序后的栈，若无进栈操作，者不用在排序

AC代码：

#include<cstdio>const int MAX = 3e5 + 10;char s[6];int a[MAX];int main(){    int n,p,m = 0,cut = 0,sum = 0;    scanf("%d",&n);    for(int i = 1; i <= n * 2; i++){        scanf("%s",s);        if(s[0] == 'a')            scanf("%d",&a[++m]);        else{            sum++;            if(m){ // 排序后栈顶进了新元素                if(a[m] == sum) m--;                else cut++,m = 0;            }        }    }    printf("%d\n",cut);    return 0;}