Basic Calculator

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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.

public class Solution {    public int calculate(String s) {        if (s == null || s.length() == 0)            return Integer.MAX_VALUE;                Stack<Integer> stack = new Stack<Integer>();        int number = 0;        int sign = 1;        int result = 0;                for (int i = 0; i < s.length(); i++) {                        char c = s.charAt(i);                        if (Character.isDigit(c)) {                number = number * 10 + (int)(c - '0');            } else if (c == '+') {                result = result + number * sign;                sign = 1;                number = 0;            } else if ( c == '-') {                result = result + number * sign;                sign = -1;                number = 0;            } else if (c == '(' ){                stack.push(result);                stack.push(sign);                result = 0;                sign = 1;            } else if (c == ')') {                result = result + sign * number;                number = 0;                result = result * stack.pop();                result = result + stack.pop();            }                                           }        if(number != 0)                result = result + number * sign;                    return result;    }}

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