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) {
        char[] charArray = s.toCharArray();
        Stack<Integer> stack = new Stack<>();
        int res = 0;
        int sign = 1;
        stack.push(1);
        for (int i = 0; i < charArray.length; i++) {
			if (Character.isDigit(charArray[i])) {
				int num = charArray[i] - '0';
				int j = i + 1;
				while (j < charArray.length && Character.isDigit(charArray[j])) {
					num = num * 10 + charArray[j] - '0';
					j++;
				}
				res += sign * stack.peek() * num;
				i = j - 1;
			} else if (charArray[i] == '+') {
				sign = 1;
			} else if (charArray[i] == '-') {
				sign = -1;
			} else if (charArray[i] == '(') {
				stack.push(sign * stack.peek());
				sign = 1;
			} else if (charArray[i] == ')') {
				stack.pop();
			} else {
				;
			}
		}
        return res;
    }
}

 

public class Solution {
    public int calculate(String s) {
        Stack<Integer> stack = new Stack<>();
        int res = 0;
        int sign = 1;
        int num = 0;
        for (int i = 0; i < s.length(); i++) {
			char ch = s.charAt(i);
			if (ch <= '9' && ch >= '0') {
				num = 10 * num + ch - '0';
			} else if (ch == '+') {
				res += sign * num;
				sign = 1;
				num = 0;
			} else if (ch == '-') {
				res += sign * num;
				sign = -1;
				num = 0;
			} else if (ch == '(') {
				stack.push(res);
				stack.push(sign);
				res = 0;
				sign = 1;
			}  else if (ch == ')') {
				res += sign * num;
				num = 0;
				res *= stack.pop();
				res += stack.pop();
			}
		}
        if (num != 0) {
        	res += sign * num;
        }
        return res;
    }
}