class Solution {

    private boolean isOperator(char op) {
        if(op == '+'  || op == '-' || op == '*' || op == '/') return true;
        return false;
    }

    private boolean isOperator(String op) {
        if(op.length()!=1) return false;
        return isOperator(op.charAt(0));
    }

    private String trim(String s) {
        StringBuilder ans = new StringBuilder();
        for(char x: s.toCharArray()) {
            if(x!=' ') ans.append(x);
        }
        return ans.toString();
    }

    public int calculate(String s) {
        s = trim(s);
        int n = s.length();
        List<String> exp = new ArrayList<>();
        int prev = 0;
        for(int i=0;i<n;i++) {
            if(isOperator(s.charAt(i))) {
                exp.add(s.substring(prev, i));
                exp.add(s.substring(i, i+1));
                prev = i+1;
            }
        }
        exp.add(s.substring(prev));

        // System.out.println(exp);

        exp = eval(exp, '*', '/');
        exp = eval(exp, '+', '-');
        return Integer.parseInt(exp.get(0));
    }


    private List<String> eval(List<String> exp, char op1, char op2) {

        Stack<String> st = new Stack<>();
        int size = exp.size();

        for(int i=0;i<size;i++) {
            String x = exp.get(i);
            if(isOperator(x) && (x.charAt(0) == op1 || x.charAt(0) == op2)) {
                String nextEle = exp.get(i+1);
                String prevEle = st.pop();
                String ans = operate(Integer.parseInt(prevEle), Integer.parseInt(nextEle), x);
                st.push(ans);
                i++;
            } else {
                st.push(x);
            }
            // System.out.println(x + " " + st);
        }

        List<String> ans = new ArrayList<>();
        while(!st.empty()) ans.add(st.pop());
        Collections.reverse(ans);
        return ans;

    }

    private String operate(int a, int b, String x) {
        if(x.equals("+")) return String.valueOf(a+b);
        if(x.equals("-")) return String.valueOf(a-b);
        if(x.equals("*")) return String.valueOf(a*b);
        if(x.equals("/")) return String.valueOf(a/b);
        return "&";
    }

}