A problem in designing a Calculator in Java

Question

I tried to solve the Problem224 on Leetcode. My code is below:

import java.util.Stack;

public class Calculator {
    public int calculate(String s){
        Stack <Integer> numst=new Stack<Integer>();
        Stack <Character> oprst=new Stack<Character>();
        Character c;
        int i;
        int number;
        int length=s.length();
        for(i=0;i<length;i++){
            c=s.charAt(i);
            switch(c){
                case ' ':
                    continue;
                case '(':
                    continue;
                case '+':
                    oprst.push('+');
                    continue;
                case '-':
                    oprst.push('-');
                    continue;
                case ')':
                    if(numst.size()==2){
                        if(oprst.peek()=='+') {
                            numst.push(numst.pop() + numst.pop());
                            oprst.pop();
                        }
                        else if(oprst.pop()=='-') {
                            numst.push(-numst.pop() + numst.pop());
                            oprst.pop();
                        }
                    }
                    else
                        continue;
                    continue;
                default:
                    if(i>0){
                        if(Character.isDigit(s.charAt(i-1))) {
                            numst.push(10 * numst.pop() + ('c' - '0'));
                        }
                        else
                            numst.push(('c'-'0'));
                    }
                    else
                        numst.push(('c'-'0'));
                    if(numst.size()==2){
                        if(oprst.peek()=='+') {
                            numst.push(numst.pop() + numst.pop());
                            oprst.pop();
                        }
                        else if(oprst.peek()=='-') {
                            numst.push(-numst.pop() + numst.pop());
                            oprst.pop();
                        }
                    }
                    else
                        continue;
                    continue;
            }
        }
        return numst.pop();
    }
}

I used two stacks. The numst stores the operands and results of every calculation. The opst stores the operator in the calculator expression.

When I put s="1 + 1" in the function, I got 102 as the answer. Can you tell me the problems in my code?