227 Basic Calculator II.cpp

static int fastio=[](){
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    return 0;
}();
class Solution {
public:
    int calculate(string s) {
        int len = s.length();
        if (len == 0) return 0;
        stack<int> stack;
        int currentNumber = 0;
        char operation = '+';
        for (int i = 0; i < len; i++) {
            char currentChar = s[i];
            if (isdigit(currentChar)) {
                currentNumber = (currentNumber * 10) + (currentChar - '0'); // for continuous number string
            }
            if (!isdigit(currentChar) && currentChar!=' ' || i == len - 1) {
                if (operation == '-') {
                    stack.push(-currentNumber);
                }
                else if (operation == '+') {
                    stack.push(currentNumber);
                }
                else if (operation == '*') {
                    int x=stack.top() * currentNumber;
                    stack.pop();
                    stack.push(x);
                }
                else if (operation == '/') {
                    int x=stack.top() / currentNumber;
                    stack.pop();
                    stack.push(x);
                }
                operation = currentChar;
                currentNumber = 0;
            }
        }
        int result = 0;
        while (!stack.empty()) {
            result += stack.top();
            stack.pop();
        }
        return result;  
    }
};