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public int calculate(String s) {
    Stack<Integer> stack = new Stack<Integer>();
    int result = 0;
    int number = 0;
    int sign = 1;
    for(int i = 0; i < s.length(); i++){
        char c = s.charAt(i);
        if(Character.isDigit(c)){
            number = 10 * number + (int)(c - '0');
        }else if(c == '+'){
            result += sign * number;
            number = 0;
            sign = 1;
        }else if(c == '-'){
            result += sign * number;
            number = 0;
            sign = -1;
        }else if(c == '('){
            //we push the result first, then sign;
            stack.push(result);
            stack.push(sign);
            //reset the sign and result for the value in the parenthesis
            sign = 1;   
            result = 0;
        }else if(c == ')'){
            result += sign * number;  
            number = 0;
            result *= stack.pop();    //stack.pop() is the sign before the parenthesis
            result += stack.pop();   //stack.pop() now is the result calculated before the parenthesis
            
        }
    }
    if(number != 0) result += sign * number;
    return result;
}

public class Solution {
    public int calculate(String s) {
        //valueStack and signStack
        //when meet sign, if next is "(" push res and sign into stacks, if next is integer, sign * integer plus res
        s = s.trim();
        if(s.length() == 0) return 0;
        Stack<Integer> valueStack = new Stack<>();
        Stack<Integer> signStack = new Stack<>();
        char[] chars = s.toCharArray();
        int i = 0, res = 0, sign = 1;
        while(i < chars.length) {
            //System.out.println(chars[i]);
            if(chars[i] == '+') {
                sign = 1;
            }
            else if(chars[i] == '-') {
                sign = -1;
            }
            else if(chars[i] >= '0' && chars[i] <= '9') {
                int j = i;
                int cur = 0;
                while(j < chars.length && chars[j] >= '0' && chars[j] <= '9') {
                    cur = cur * 10 + chars[j] - '0';
                    j++;
                }
                i = --j;
                //System.out.println(cur);
                res += sign * cur;
            }
            else if(chars[i] == '(') {
                valueStack.push(res);
                signStack.push(sign);
                res = 0;
                sign = 1;
            }
            else if(chars[i] == ')') {
                res = res * signStack.pop() + valueStack.pop();
            }
            else if(chars[i] == ' ') {
                i++;
                continue;
            }
            i++;
        }
        return res;
    }
}

public class Solution {
    private int getResult(int a, int b, char op) {
        switch(op) {
            case '+' : return a + b;
            default : return a - b;
        }
    }
    public int calculate(String s) {
        if (s == null || s.length() < 1) return 0;
        Stack<Character> operators = new Stack<>();
        Stack<Integer> operands = new Stack<>();
        char[] str = s.toCharArray();
        int len = str.length;
        int num = 0;
        int i = 0;
        while (i < len) {
            if (str[i] == ' ') {
                i++;
                continue;
            }
            if (str[i] >= '0' && str[i] <= '9') {
                num = str[i] - '0';
                i++;
                while (i < len && str[i] >= '0' && str[i] <= '9') {
                    num = num * 10 + str[i] - '0';
                    i++;
                }
                operands.push(num);
                num = 0;
            } else if (str[i] == '(') {
                operators.push(str[i++]);
            } else if (str[i] == ')') {
                while (operators.peek() != '(') {
                    int second = operands.pop();
                    int first = operands.pop();
                    operands.push(getResult(first, second, operators.pop()));
                }
                operators.pop();
                i++;
            } else if (str[i] == '+' || str[i] == '-') {
                while (!operators.isEmpty() && operators.peek() != '(') {
                    int second = operands.pop();
                    int first = operands.pop();
                    operands.push(getResult(first, second, operators.pop()));
                }
                operators.push(str[i++]);
            } 
        }
        while (!operators.isEmpty()) {
            int second = operands.pop();
            int first = operands.pop();
            operands.push(getResult(first, second, operators.pop()));
        }
        return operands.pop();
    }
}

public class Solution {
    public int calculate(String s) {
        if (s == null || s.length() == 0) return 0;
        Stack<Integer> stack = new Stack<>();
        int res = 0;//curr result
        int sign = 1;//sign: the previous sign for curr num. initialize it as 1 is important!
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (Character.isDigit(c)) {
                int sum = c - '0';//sum up the whole num
                while (i + 1 < s.length() && Character.isDigit(s.charAt(i + 1))) {//s.charAt(i + 1), not i + 1 !!!
                    sum = sum * 10 + s.charAt(i + 1) - '0';//see how to get the curr num
                    i++;
                }
                res += sum * sign;//add the sum to res
            } else if (c == '+') {
                sign = 1;
            } else if (c == '-') {
                sign = -1;
            } else if (c == '(') {
                stack.push(res);
                stack.push(sign);//sign is on the top
                res = 0;//update res to 0, which is the sum in the ( )
                sign = 1;
            } else if (c == ')') {
                res = res * stack.pop() + stack.pop();//first pop is the sign, second is the previous sum before ( )
            }
        }
        return res;
    }
}