224.BasicCalculator.cpp

#include<bits/stdc++.h>
using namespace std;

class Solution {
public:
    int calculate(string s) {
         stack<int> nums; 
        int answer=0; 
        long long current_val=0, sign=1; 
       
        
     for (char c : s) { 
            if (isdigit(c)) {
         current_val=10*current_val + c-'0'; 
            }else if(c=='+'){
                answer+=current_val*sign; 
                current_val=0;
                sign=1;   
            }else if(c=='-'){
                answer+=current_val*sign; 
                current_val=0; 
                sign=-1; 
            }else if(c=='('){
                nums.push(answer);    
                nums.push(sign); 
                answer=0; 
                sign=1;
            }else if(c==')' ){ 
                answer+=current_val*sign; 
                current_val=0;
                
                answer = answer*nums.top();
                nums.pop();
                answer= answer+nums.top();
                nums.pop();
            }
        }
        answer+=current_val*sign; 
        return answer; 
    }
};