标签：std log logs air cout blog get pac cin
题目：http://www.51nod.com/onlineJudge/questionCode.html#!problemId=1305
统计1和2的数量
1-1是2 1-x（不为1）是1 2-2是1
时间复杂度O(n)
#include<iostream> using namespace std; const int maxn=1e5+5; int a[maxn],n,n1=0,n2=0; int main() { __int64 sum; cin>>n; for(int i=0;i<n;i++) { cin>>a[i]; if(a[i]==1) n1++; else if(a[i]==2) n2++; } int n11=n1*(n1-1)/2,n1x=n1*(n-n1),n22=n2*(n2-1)/2; sum=n11*2+n22+n1x; cout<<sum; }
标签：std log logs air cout blog get pac cin
原文地址：http://www.cnblogs.com/NDKY9/p/7608661.html
