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"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23

java code:

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

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