Basic Calculator

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:
“1 + 1” = 2
” 2-1 + 2 ” = 3
“(1+(4+5+2)-3)+(6+8)” = 23
Note: Do not use the eval built-in library function.

程序转自讨论区：https://leetcode.com/discuss/39553/iterative-java-solution-with-stack

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;}