[LeetCode] Problem 224 - Basic Calculator

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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 .

Example

No.1

Input: “1 + 1”

Output: 2

No.2

Input: “ 2-1 + 2 “

Output: 3

No.3

Input: “(1+(4+5+2)-3)+(6+8)”

Output: 23

Note

  • You may assume that the given expression is always valid.
  • Do not use the eval built-in library function.

Code

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public int calculate(String s) {
    Stack<Integer> stack = new Stack<>();
    int result = 0;
    int num = 0;
    int sign = 1;

    for (int i = 0; i < s.length(); i++) {
        char ch = s.charAt(i);

        if (Character.isDigit(ch)) {
            num = 10 * num + (ch - '0');
        }
        else if (ch == '+' || ch == '-') {
            result += sign * num;
            num = 0;
            sign = ch == '+' ? 1 : -1;
        }
        else if (ch == '(') {
            stack.push(result);
            stack.push(sign);
            sign = 1;
            result = 0;
        }
        else if (ch == ')') {
            result += sign * num;
            num = 0;
            result *= stack.pop();
            result += stack.pop();
        }
    }

    result += sign * num;
    return result;
}