Basic Calculator

224. 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.

class Solution {
    func calculate(_ s: String) -> Int {
      if s.count == 0 {
        return 0
      }
      
      var stack: [Int] = []
      var sign = 1
      var result = 0
      
      var i = 0
      while i < s.count {
        let c = s[s.index(s.startIndex, offsetBy: i)]
        
        if isDigit(c) {
          var num = Int(String(c))!
          
          while i < s.count - 1 
               && isDigit(s[s.index(s.startIndex, offsetBy: i + 1)]) {
            num = num * 10 
                 + Int(String(s[s.index(s.startIndex, offsetBy: i + 1)]))!
            i += 1
          }
          
          result += num * sign
        } else if c == "+" {
          sign = 1  
        } else if c == "-" {
          sign = -1
        } else if c == "(" {
          stack.append(result)
          stack.append(sign)
          result = 0
          sign = 1
        } else if c == ")" {
          result = result * stack.removeLast() + stack.removeLast()  
        }
        
        i += 1
      }
      
      return result
    }

  func isDigit(_ c: Character) -> Bool {
    if let val = Int(String(c)) {
      if val >= 0 && val <= 9 {
        return true
      }
    }
        
    return false
  }
}

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

Hope this helps,
Michael