公式E（x）=n Σ 1/i
因为当已经拿到k张不同的时候 拿到不同牌的期望是 （n-k）/n ,1除于这个概率就是 n/(n-k) 然后从1到n加
#include<iostream>#include<cstring>#include<string>#include<algorithm>#include<cstdio>#include<cassert>using namespace std;typedef long long ll;ll gcd(ll a,ll b){return b==0?a:gcd(b,a%b);}struct Fraction{long long num;long long den;Fraction(long long num=0,long long den=1){if(den<0){num=-num;den=-den;}long long g=gcd(abs(num),den);this->num=num/g;this->den=den/g;}Fraction operator +(const Fraction &o)const{return Fraction(num*o.den+den*o.num,den*o.den);}Fraction operator -(const Fraction &o)const{return Fraction(num*o.den-den*o.num,den*o.den);}Fraction operator *(const Fraction &o)const{return Fraction(num*o.num,den*o.den);}Fraction operator /(const Fraction &o)const{return Fraction(num*o.den,den*o.num);}bool operator <(const Fraction &o)const{return num*o.den<den*o.num;}bool operator ==(const Fraction &o)const{return num*o.den==den*o.num;}};int n;int main(){while(cin>>n){Fraction ans(n,1);Fraction tmp(0,1);for(int i=1;i<=n;i++)tmp=tmp+Fraction(1,i);ans=ans*tmp;long long a,b,c;a=ans.num;b=ans.den;if(a/b!=0){long long gd=gcd(a,b);a/=gd;b/=gd;c=a/b;a=a-c*b;}elsec=0;if(a==0)printf("%lld\n",c);else{int l1,l2,l3;long long t1=c,t2=a,t3=b;l1=0;while(t1){t1/=10;l1++;}l2=0;while(t2){t2/=10;l2++;}l3=0;while(t3){t3/=10;l3++;}if(l1!=0){for(int i=0;i<=l1;i++) printf(" ");printf("%lld\n",a);printf("%lld ",c);int len=max(l2,l3);for(int i=0;i<len;i++) printf("-");printf("\n");for(int i=0;i<=l1;i++) printf(" ");printf("%lld\n",b);}else{printf("%I64d\n",a);int len=max(l2,l3);for(int i=0;i<len;i++)printf("-");printf("\n");printf("%I64d\n",b);}}}return 0;}
，看天，看雪，安安静静，不言不语都是好风景。
