♥CodeForces 609B-B. The Best Gift【组合数学】

Myth丶恋晨 2022-08-19 11:29 141阅读 0赞

B. The Best Gift

time limit per test 2 seconds

memory limit per test 256 megabytes

input standard input

output standard output

Emily's birthday is next week and Jack has decided to buy a present for her. He knows she loves books so he goes to the local bookshop, where there are*n* books on sale from one of *m* genres.

In the bookshop, Jack decides to buy two books of different genres.

Based on the genre of books on sale in the shop, find the number of options available to Jack for choosing two books of different genres for Emily. Options are considered different if they differ in at least one book.

The books are given by indices of their genres. The genres are numbered from  1 to *m*.

Input

The first line contains two positive integers *n* and*m* (2 ≤ *n* ≤ 2·105, 2 ≤ *m* ≤ 10) — the number of books in the bookstore and the number of genres.

The second line contains a sequence *a*1, *a*2, ..., *a**n*, where*a**i* (1 ≤ *a**i* ≤ *m*) equals the genre of the*i*\-th book.

It is guaranteed that for each genre there is at least one book of that genre.

Output

Print the only integer — the number of ways in which Jack can choose books.

It is guaranteed that the answer doesn't exceed the value  2·109.

Sample test(s)

Input

4 3
    2 1 3 1

Output

Input

7 4
    4 2 3 1 2 4 3

Output

解题思路：

题目大意是抽取两本书，每个位置上的相同类型的书是不一样的，第一组测试数据中的第二个和第四个就不行，我们应该选取两个不同种类的。暴力肯定不行，用组合数学公式，算会超 long long ，所以介绍下面算法。

#include<stdio.h>
    #include<string.h>
    #include<algorithm>
    using namespace std;
    int num[100000]; 
    int main()
    {
    	int n,m;
    	while(scanf("%d%d",&n,&m)!=EOF)
    	{
    		int i,j;
    		int ans=0;
    		memset(num,0,sizeof(num));
    		for(i=0;i<n;i++)
    		{
    			int x;
    			scanf("%d",&x);
    			ans+=i-num[x];
    			num[x]++;
    		}
    		printf("%d\n",ans);
    	}
    	return 0;
    }