Decode String

public class Solution {
    /**
     * @param s: an expression includes numbers, letters and brackets
     * @return: a string
     */
    public String expressionExpand(String s) {
        Stack<Object> stack = new Stack<>();
        int num = 0;
        
        for (char c: s.toCharArray()) {
            if (Character.isDigit(c)) {
                num = num * 10 + c - ‘0‘;
            } else if (c == ‘[‘) {
                stack.push(Integer.valueOf(num));
                num = 0;
            } else if (c == ‘]‘) {
                String newStr = popStack(stack);
                Integer count = (Integer) stack.pop();
                for (int i = 0; i < count; i++) {
                    stack.push(newStr);
                }
            } else {
                stack.push(String.valueOf(c));
            }
            
        }
        return popStack(stack);
    }
    
    private String popStack(Stack<Object> stack) {
        // pop stack until stack is empty or get a number
        Stack<String> buffer = new Stack<>();
        while (!stack.isEmpty() && (stack.peek() instanceof String)) {
            buffer.push((String) stack.pop());
        }
        
        StringBuilder sb = new StringBuilder();
        while (!buffer.isEmpty()) {
            sb.append(buffer.pop());
        }
        return sb.toString();
    }
}