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
public class Solution {
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;
}
}