basic-calculator-ii.java

// Problem link - https://leetcode.com/problems/basic-calculator-ii/

// TC - O(N)
// SC - O(N)
class Solution {
    public int calculate(String s) {
        Stack<Integer> st = new Stack<>();

        int num = 0;
        char operator = '+';

        for(int i = 0; i < s.length(); i++){
            char c = s.charAt(i);

            // 0-9
            if(Character.isDigit(c)){
                num = num * 10 + (c - '0');
            }

            if(isOperator(c) || i == s.length() - 1){
                if(operator == '+') st.push(num);
                else if (operator == '-') st.push(-num);
                else if (operator == '*') st.push(st.pop() * num);
                else if (operator == '/') st.push(st.pop() / num);

                num = 0;
                operator = c;
            }
        }

        int ans = 0;

        while(!st.isEmpty()){
            ans += st.pop();
        }

        return ans;
    }

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