Time Limit: 3000MS | Memory Limit: 65536K | |
Total Submissions: 2476 | Accepted: 800 |
Description
For every pair of triplets, Ta = (Ia, Ja, Ka) and Tb = (Ib, Jb, Kb), we define the difference value between Ta and Tb as follows:
D(Ta, Tb) = max {Ia − Ib, Ja − Jb, Ka − Kb} − min {Ia − Ib, Ja − Jb, Ka − Kb}
Now you are given N triplets, could you write a program to calculate the sum of the difference values between every unordered pair of triplets?Input
Each test case begins with a line containing an integer N, denotes the number of triplets. Assume that we number the triplets as T1, T2, ... , TN. Then, there are following N lines, each line contains three integers, giving the elements of each triplet.
A case with N = 0 indicates the end of the input.
Output
Sample Input
2 1 2 3 3 2 1 3 1 3 2 4 0 7 2 2 9 0
Sample Output
4 20
Hint
Case 2: D(T1,T2)+D(T1,T3)+D(T2,T3)=8+8+4=20
You can assume that N, the number of triplets in each case, will not exceed 200,000 and the elements in triplets fit into [-106,106].
The size of the input will not exceed 5 MB.
Source
题目的意思很easy理解,我就不多说了。
用n^2算法必跪的啦,必需要O(n)
先要理解max{a,b,c}-min{a,b,c}=(|a-b|+|b-c|+|c-a|)/2
如果a>b>c,那么max{a-b-c}-min{a-b-c}=a-c
而(|a-b|+|b-c|+|c-a|)/2=(a-b+b-c+c-a)/2=a-c=max{a-b-c}-min{a-b-c}
而由位置的对称性,不管a,b,c关系怎样,我们总能够交换其似的a',b',c'保持a'>b'>c',同一时候结果总是一致。
而| (Ia-ka) -(Ib-kb)| = |(Ia-Ib)-(ka-kb)|
所以我们对于Ti=(Ia,Ib,Ic)能够先计算Ia-Ib,Ib-Ic,Ic-Ia
这样我们要计算D(Ti,Tj),则是求(|a-b|+|b-c|+|c-a|)/2.能够保持线性。仅仅要保证a>b, b>c, c>a,则能够将绝对值去掉。
进行排序,排序后a[i],b[i],c[i],a[i]做头的是i次,被减的是n-i-1次(从0起的坐标)
这样,ans=sum(i*(a[i]+b[i]+c[i])-(n-i-1)*(a[i]+b[i]+c[i]))
具体见代码。
思路的确比較坑。
#include <iostream> #include <algorithm> using namespace std; #define N 200005 long long a[N],b[N],c[N]; int main() { int i,n; long long x,y,z; cin.sync_with_stdio(false); while(cin>>n,n){ for(i=0;i<n;i++){ cin>>x>>y>>z; a[i]=x-y,b[i]=y-z,c[i]=z-x; } sort(a,a+n); sort(b,b+n); sort(c,c+n); long long ans=0LL; for(i=0;i<n;i++){ ans+=(i*(a[i]+b[i]+c[i])-(n-i-1)*(a[i]+b[i]+c[i])); } cout<<ans/2<<endl; } return 0; }