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LeetCode

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Problem

Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.

Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

Example 1:

Input: s = "1 + 1"
Output: 2

Example 2:

Input: s = " 2-1 + 2 "
Output: 3

Example 3:

Input: s = "(1+(4+5+2)-3)+(6+8)"
Output: 23

Code

class Solution {
    public int calculate(String s) {
        if(s == null || s.length() == 0) return 0;

        Stack<String> stack = new Stack<>();

        String sign = "+";
        int i = 0;
        while(i < s.length()){
            char c = s.charAt(i);

            if(c ==' '){
                i++;
                continue;
            }else if(c == '('){
                stack.push(sign);
                stack.push("(");

                sign="+";
            } else if (c == ')'){
                int temp = 0;

                while(stack.peek() != "("){
                    temp += Integer.valueOf(stack.pop());
                }

                stack.pop();
                if(stack.pop() == "+"){
                    stack.push(temp + "");
                } else {
                    stack.push(-temp + "");
                }

            } else if(c == '+'){
                sign = "+";
            } else if (c == '-'){
                sign = "-";
            } else {
                int temp = c - '0';
                while(i + 1 < s.length() && Character.isDigit(s.charAt(i + 1))){
                    temp = temp * 10 + (s.charAt(i + 1) - '0');
                    i++;
                }

                if(sign == "+"){
                    stack.push(temp + "");
                } else {
                    stack.push(-temp + "");
                }
            }

            i++;
        }

        int res = 0;
        while(!stack.isEmpty()){
            res +=  Integer.valueOf(stack.pop());
        }

        return res;
    }
}