uva11039 设计建筑物

题目：
Input
The input file consists of a first line with the number p of cases to solve. The first line of each case
contains the number of available floors. Then, the size and colour of each floor appear in one line.
Each floor is represented with an integer between -999999 and 999999. There is no floor with size 0.
Negative numbers represent red floors and positive numbers blue floors. The size of the floor is the
absolute value of the number. There are not two floors with the same size. The maximum number of
floors for a problem is 500000.
Output
For each case the output will consist of a line with the number of floors of the highest building with
the mentioned conditions.
Sample Input
2
5
7
-2
6
9
-3
8
11
-9
2
5
18
17
-15
4
Sample Output
2
5

题意：
有n个绝对值各不相同的非0整数，选出尽可能多的数，排成一列，使得正负号交替，且绝对值递增。输入整数n和n个整数，输出最长序列长度。

分析：
先依照绝对值从小到大排序，记录选出的序列最后一个的符号，正负交替时ans加1.

ac代码：

#include<iostream>#include<algorithm>#include<cstdio>#include<cstring>#include<cmath>using namespace std;const int maxn=5e5+5;int a[maxn];bool cmp(int a,int b){    return abs(a)<abs(b);}int main(){    //freopen("test.txt","r",stdin);    int cas;    scanf("%d",&cas);    while(cas--)    {        int n;        scanf("%d",&n);        memset(a,0,sizeof(a));        for(int i=1;i<=n;i++)        scanf("%d",&a[i]);        sort(a+1,a+n+1,cmp);        int change,ans=1;        if(a[1]<0)        change=-1;        else        change=1;        for(int i=2;i<=n;i++)        {            if(a[i]*change<0)            {                ans++;                change=-change;            }        }        printf("%d\n",ans);    }}