需要注意的几个点：

• 当输入出现混合情况，尤其是字符的时候，需要根据情况分析，是否需要加空格，例如本题目中，输入为 a b +，其中加号前面有空格，需要缓冲掉，使用scanf("%d%d %c",&a,&b,&c); 否则会将空格赋值给变量c。
• 输出格式为运算表达式的中缀形式以及向零取整的结果，中间用空格隔开。
• $a,b\in [-10^9,10^9], a*b\in [-10^{18},10^{18}]$，需要开 long long.

C语言

#include<stdio.h>
int main() {
	int a,b; char c;
	scanf("%d%d %c",&a,&b,&c);
	switch (c) {
		case '+':printf("%d + %d = %d",a,b,a+b); break;
		case '-':printf("%d - %d = %d",a,b,a-b); break;
		case '*':printf("%d * %d = %lld",a,b,(long long)a*b); break;
		case '/':
                if(b==0) printf("%d / %d = Divided by zero!",a,b);
                else printf("%d / %d = %d",a,b,a/b);
                break;
		default:printf("%d %c %d = Invalid operator!", a, c, b);
	} 
	return 0;
}

C++

可以考虑先输出中缀表达式后接 = 的内容，之后在输出结果，这样不容易出错。

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 1e6 + 10, INF = 0x3f3f3f3f;

int main() {
    int a, b; char c;
    cin>>a>>b>>c;
    cout<<a<<" "<<c<<" "<<b<<" = ";
    if (c == '+') cout<<a+b<<endl;
    else if (c == '-') cout<<a-b<<endl;
    else if (c == '*')cout<<1ll*a*b<<endl;
    else if (c == '/') {
        if (b == 0) cout<<"Divided by zero!\n";
        else cout<<a/b<<endl;
    } else {
        cout<<"Invalid operator!\n";
    }
    return 0;
}

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextLong();
        char c = sc.next().charAt(0);
        switch(c){
            case '+': System.out.println(a + " + " + b + " = " + (a+b)); break;
            case '-': System.out.println(a + " - " + b + " = " + (a-b)); break;
            case '*': System.out.println(a + " * " + b + " = " + (a*b)); break;
            case '/':
                if(b == 0){
                    System.out.println(a + " / " + b + " = " + "Divided by zero!");
                }else{
                    System.out.println(a + " / " + b + " = " + (a/b));
                }
                break;
            default:
                System.out.println(a + " "+c+" " + b + " = " + "Invalid operator!");
                break;
        }
    }
}