Expression evaluation (stack)

#include <iostream>
#include <cstring>
#include <algorithm>
#include <unordered_map>
#include <stack>

using namespace std;

stack<char> op;
stack<int> num;

void eval()
{
    
    auto b = num.top(); num.pop();
    auto a = num.top(); num.pop();
    auto c = op.top(); op.pop();

    int x;
    if (c == '+') x = a + b;
    else if (c == '-') x = a - b;
    else if (c == '*') x = a * b;
    else x = a / b;
    num.push(x);
}

int main()
{
    
    string s;
    cin >> s;

    unordered_map<char, int> pr{
    {
    '+', 1}, {
    '-', 1}, {
    '*', 2}, {
    '/', 2}};
    for (int i = 0; i < s.size(); i ++ )
    {
    
        if (isdigit(s[i]))
        {
    
            int j = i, x = 0;
            while (j < s.size() && isdigit(s[j]))
                x = x * 10 + s[j ++ ] - '0';
            num.push(x);
            i = j - 1;
        }
        else if (s[i] == '(') op.push(s[i]);
        else if (s[i] == ')')
        {
    
            while (op.top() != '(') eval();
            op.pop();
        }
        else
        {
    
            while (op.size() && op.top() != '(' && pr[op.top()] >= pr[s[i]])
                eval();
            op.push(s[i]);
        }
    }

    while (op.size()) eval();
    cout << num.top() << endl;

    return 0;
}
/*  author ：yxc  link ：https://www.acwing.com/activity/content/code/content/1431326/  source ：AcWing  The copyright belongs to the author . Commercial reprint please contact the author for authorization , Non-commercial reprint please indicate the source .*/

Infix expression to suffix expression

#include <iostream>
#include <cstring>
#include <algorithm>
#include <unordered_map>
#include <stack>

using namespace std;

stack<char> op;

void eval()
{
    
    auto c = op.top(); op.pop();
    cout << c << ' ';
}

int main()
{
    
    string s;
    cin >> s;

    unordered_map<char, int> pr{
    {
    '+', 1}, {
    '-', 1}, {
    '*', 2}, {
    '/', 2}};
    for (int i = 0; i < s.size(); i ++ )
    {
    
        if (isdigit(s[i]))
        {
    
            int j = i, x = 0;
            while (j < s.size() && isdigit(s[j]))
                x = x * 10 + s[j ++ ] - '0';
            cout << x << ' ';
            i = j - 1;
        }
        else if (s[i] == '(') op.push(s[i]);
        else if (s[i] == ')')
        {
    
            while (op.top() != '(') eval();
            op.pop();
        }
        else
        {
    
            while (op.size() && op.top() != '(' && pr[op.top()] >= pr[s[i]])
                eval();
            op.push(s[i]);
        }
    }

    while (op.size()) eval();

    return 0;
}
/*  author ：yxc  link ：https://www.acwing.com/activity/content/code/content/1431326/  source ：AcWing  The copyright belongs to the author . Commercial reprint please contact the author for authorization , Non-commercial reprint please indicate the source .*/