BasicCalculatorII.java

class BasicCalculatorII {
    // TC => O(n), Space O(n)
    public int calculate(String s) {
        Stack<Integer> st = new Stack<>();

        char prevSign = '+';
        int no = 0;
        for(int i=0;i<s.length();i++){
            char c = s.charAt(i);
            if(Character.isDigit(c)){
                no = no*10  + (c-'0');
            }

            if(isOperator(c) || i ==s.length()-1){
                if(prevSign == '+'){
                    st.push(no);
                } else if(prevSign == '-'){
                    st.push(-no);
                } else if(prevSign =='*'){
                    int top = st.pop();

                    st.push(top * no);

                } else if(prevSign =='/'){
                    int top = st.pop();
                    st.push(top/no);
                }
                no =0;
                prevSign =c;
            }
        }

        int res = 0;
        while(!st.empty()){ // 1+2+3+4
            res += st.pop();
        }
        return res;
    }

    private boolean isOperator(char c){
        return c=='+' || c=='-'|| c=='/' || c=='*';
    }
}