Elegant Construction
题目连接：
http://acm.hdu.edu.cn/showproblem.php?pid=5813
Description
Being an ACMer requires knowledge in many fields, because problems in this contest may use physics, biology, and even musicology as background. And now in this problem, you are being a city architect!
A city with N towns (numbered 1 through N) is under construction. You, the architect, are being responsible for designing how these towns are connected by one-way roads. Each road connects two towns, and passengers can travel through in one direction.
For business purpose, the connectivity between towns has some requirements. You are given N non-negative integers a1 .. aN. For 1 <= i <= N, passenger start from town i, should be able to reach exactly ai towns (directly or indirectly, not include i itself). To prevent confusion on the trip, every road should be different, and cycles (one can travel through several roads and back to the starting point) should not exist.
Your task is constructing such a city. Now it's your showtime!
Input
The first line is an integer T (T <= 10), indicating the number of test case. Each test case begins with an integer N (1 <= N <= 1000), indicating the number of towns. Then N numbers in a line, the ith number ai (0 <= ai < N) has been described above.
Output
For each test case, output "Case #X: Y" in a line (without quotes), where X is the case number starting from 1, and Y is "Yes" if you can construct successfully or "No" if it's impossible to reach the requirements.
If Y is "Yes", output an integer M in a line, indicating the number of roads. Then M lines follow, each line contains two integers u and v (1 <= u, v <= N), separated with one single space, indicating a road direct from town u to town v. If there are multiple possible solutions, print any of them.
Sample Input
3
3
2 1 0
2
1 1
4
3 1 1 0
Sample Output
Case #1: Yes
2
1 2
2 3
Case #2: No
Case #3: Yes
4
1 2
1 3
2 4
3 4
Hint
题意
给你n个城市，告诉你第i个城市恰好能够走到a[i]个城市，让你构造一个有向图，使得满足题意，且不存在环。
题解：
直接暴力去建图就好了，n^2扫一遍，然后只扫编号比自己小的，这样就不会存在环了。
代码
#include<bits/stdc++.h>
using namespace std;
const int maxn = 1005;
struct node{
int a,id;
}p[1005];
bool cmp(node a,node b){
return a.a<b.a;
}
int cas = 0;
int ansx[maxn*maxn],ansy[maxn*maxn];
void solve(){
printf("Case #%d: ",++cas);
int sum = 0;
int cnt=0;
int n;
scanf("%d",&n);
for(int i=1;i<=n;i++)
scanf("%d",&p[i].a),p[i].id=i;
sort(p+1,p+1+n,cmp);
for(int i=1;i<=n;i++){
if(p[i].a>=i){
printf("No
");
return;
}
for(int j=1;j<=p[i].a;j++)
ansx[cnt]=p[i].id,ansy[cnt++]=p[j].id;
}
printf("Yes
");
printf("%d
",cnt);
for(int i=0;i<cnt;i++){
printf("%d %d
",ansx[i],ansy[i]);
}
}
int main(){
//freopen("1.txt","r",stdin);
int t;
scanf("%d",&t);
while(t--)solve();
}