[LeetCode 227] Basic Calculator II

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7
" 3/2 " = 1
" 3+5 / 2 " = 5

Note: Do not use the eval built-in library function.

solution:

we need mark + - * / with different Integer

fist to calculate * and / , then left only numbers and + - in list.

second to calculate rest

public int calculate(String s) {
        if(s == null || s.isEmpty()) return 0;
        LinkedList<Integer> numbers = new LinkedList<>();
        for(int i=0;i<s.length();i++){
            char c = s.charAt(i);
            if(c == ' ') continue;
            if(c>='0'&& c<='9'){
                int cur = c - '0';
                while(i+1< s.length() && Character.isDigit(s.charAt(i+1))){
                    cur = 10*cur + s.charAt(++i) - '0';
                }
                if(!numbers.isEmpty()&& (numbers.getLast()==2 || numbers.getLast()==3)){
                    int op1 = numbers.pollLast();
                    int op2 = numbers.pollLast();
                    int res = 0;
                    if(op1 == 2 ){
                        res = op2 * cur;
                    }else {
                        res = op2 / cur;
                    }
                    numbers.add(res);
                }else {
                    numbers.add(cur);
                }
            }else {
                switch (c) {
                case '+':
                    numbers.add(0);
                    break;
                case '-':
                    numbers.add(1);
                    break;
                case '*':
                    numbers.add(2);
                    break;
                case '/':
                    numbers.add(3);
                    break;
                }
            }
        }
        if(numbers.size() == 1) return numbers.peek();
        int res = numbers.pop();
        while(!numbers.isEmpty()){
            int op1 = numbers.pop();
            int op2 = numbers.pop();
            if(op1 == 0){
                res = res + op2;
            }else {
                res = res - op2;
            }
        }
        return res;
    }