USACO2005 City Skyline /// oj23401

题目大意：

Input

* Line 1: Two space separated integers: N and W

* Lines 2..N+1: Two space separated integers, the x and y coordinate of a point where the skyline changes. The xcoordinates are presented in strictly increasing order, and the first x coordinate will always be 1.

Output

* Line 1: The minimum number of buildings to create the described skyline.

Sample Input

10 26
1 1
2 2
5 1
6 3
8 1
11 0
15 2
17 3
20 2
22 1

Sample Output

6

Hint

INPUT DETAILS:

The case mentioned above

需要找到规律

题解：
模拟一个单调栈，使栈内的元素递增。
当新进来一个元素x时，while栈顶的元素大于当前x,ans++，将栈顶元素弹出
如果栈顶元素等于x则只保留一个，ans不变
最后加进去一个0元素把栈按照上述方式清空即可

#include <bits/stdc++.h>
using namespace std;
int main()
{
        int n,w;
        while(~scanf("%d%d",&n,&w))
        {
            stack <int> s;
            while(!s.empty()) s.pop();
            int a,b,ans=0;
            for(int i=0;i<=n;i++)
            {
                if(i==n) b=0;
                else scanf("%d%d",&a,&b);
                if(!s.empty())
                {
                        while(!s.empty()&&b<s.top())
                        {
                            ans++;
                            s.pop();
                        }
                        if(!s.empty()&&b==s.top()) s.pop();
                        s.push(b);
                }
                else s.push(b);
            }
            printf("%d
",ans);
        }

        return 0;
}