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;
}