0分代码100Ac代码，已验证，放心复制↓

#include <iostream>
using namespace std;
int main()
{
	int a;
	cin >> a;
	if(90<=a && a<=100)
		cout << "A";
	else if(80<=a && a<=89)
		cout << "B";
	else if(70<=a && a<=79)
		cout << "C";
	else if(60<=a && a<=69)
		cout << "D";
	else if(a<60)
		cout << "E";
	
    return 0;
}

#include <iostream>
using namespace std;
int a;
int main()
{
    cin >> a;
    if (a >= 90)
        cout << 'A';
    else if (a >= 80)
        cout << 'B';
    else if (a >= 70)
        cout << 'C';
    else if (a >= 60)
        cout << 'D';
    else
        cout << 'E';
    return 0;
}

咕咕嘎嘎

#include <bits/stdc++.h>//万能头文件
using namespace std;//命名空间
int a;
int main(){//主函数入口
	cin>>a;
	
	if(a>100) return 0;//主函数出口
	
	
	if(a>=90){
		cout<<"A";
		return 0;//主函数出口
	}  
	
	
	if(a>=80&&a<=89){
		cout<<"B";
		return 0;//主函数出口
	}
	
	
	if(a>=70&&a<=79){
		cout<<"C";
		return 0;//主函数出口
	} 
		
	
	if(a>=60&&a<=69){
		cout<<"D";
		return 0;//主函数出口
	} 
	
	
	else cout<<"E";
	return 0;//主函数出口
}	

最难的代码 最简洁的代码

#include<iostream>
using namespace std;
int main ()
{
	int a;
	cin >> a;
	if(a>=90)
	{
		cout << "A";
	}
	
	if(a>=80&a<90)
	{
		cout << "B";
	}
	if(a>=70&a<80)
	{
		cout << "C";
	 } 
	 if(a>=60&a<70)
	{
		cout << "D";
	}
	if(a<60)
	{
		cout << "E";
	}
}

#include <queue>
#include <math.h>
#include <stack>
#include <stdio.h>
#include <iostream>
#include <vector>
#include <iomanip>
#include <string.h>
#include <algorithm>
using namespace std;
#define elif else if
#define int long long
#define float double
#define N 30000010
#define INF 0xc00000fd
#define scf scanf
#define ptf printf
#define gtc getchar
#define ptc putchar
#define psp ptc(' ')
#define pel ptc('\n')
#define pzr ptc('0')
#define fin for(int i = 1; i <= n; i++)
#define fim for(int i = 1; i <= m; i++)
#define fjn for(int j = 1; j <= n; j++)
#define fjm for(int j = 1; j <= m; j++)
#define rai fin	a[i] = read()
#define wct write(cnt)
#define pct print(cnt)
#define was write(ans)
#define pas print(ans)
queue<int> Q;
stack<int> S;
vector<int> V;
inline int wei(int n){
	int cnt = 0;
	while(n > 0){
		n /= 10;
		cnt++;
	}
	return cnt;
}
inline bool isPrime(int n){
	if(n < 2)
		return 0;
	for(int i = 2; i * i <= n; ++i)
		if(n % i == 0)
			return 0;
	return 1;
}
inline int read(string n){
	int x = 0, f = 1;
	ptf("%s", n.c_str());
	char c = gtc();
	while(c < '0'  ||  c > '9'){
		if(c == '-')
			f = -1;
		c = gtc();
	}
	while(c >= '0'  &&  c <= '9'){
		x = x * 10 + c - 48;
		c = gtc();
	}
	return x * f;
}
inline float input(string n){
	float x = 0, f = 1, x2 = 0, cnt = 0, i = 0;
	ptf("%s", n.c_str());
	char c = gtc();
	while(c < '0'  ||  c > '9'){
		if(c == '-')
			f = -1;
		c = gtc();
	}
	while(c >= '0'  &&  c <= '9'){
		x = x * 10 + c - 48;
		c = gtc();
	}
	c = gtc();
	while(c >= '0'  &&  c <= '9'){
		x2 = x2 * 10 + c - 48;
		cnt++;
		c = gtc();
	}
	for(; i < cnt; i++)
		x2 /= 10.0;
	return (x + x2) * f;
}
inline void write(int n) {
    if(n < 0){
        ptc('-');
        n = -n;
    }
    if(n > 9)
		write(n / 10);
    ptc(n % 10 + '0');
	return;
}
inline void print(int n, float m){
	write(m);
	m -= (int)m;
	ptc('.');
	int cnt = 0;
	while(m - (int)m && n - cnt){
		m *= 10;
		ptc((int)m % 10 + '0');
		cnt++;
	}
	n -= cnt;
	fin
		ptc('0');
	return;
}
#define input() input("")
#define read() read("")
int n = read();
char c;
inline void Main(){
	if(n > 90)
		c = 'A';
	elif(n >= 80)
		c = 'B';
	elif(n >= 70)
		c = 'C';
	elif(n >= 60)
		c = 'D';
	else
		c = 'E';
	ptc(c);
	return;
}
signed main(signed argc, char **argv){
	Main();
	pel;
	return 0;
}

#include <iostream>
using namespace std;
int main(){
    int n;
    cin>>n;
    if(n>=90)
    cout<<"A";
    else if(n>=80)
        cout<<"B";
    else if(n>=70)
        cout<<"C";
    else if(n>=60)
        cout<<"D";
    else    
        cout<<"E";
    return 0;
}