394. Decode String

class Solution {
    public String decodeString(String s) {
    Stack<Character> stack = new Stack<>();
        for(char ch : s.toCharArray()){
            if(ch != ']'){
                stack.push(ch);
            }else{
                //get the sub string
                StringBuilder sb = new StringBuilder();
                while(stack.peek() != '['){
                    sb.append(stack.pop());
                }
                //remove the '[' character
                stack.pop();
                //get the number
                int k = 0;
                int base = 1;
                while(!stack.isEmpty() && Character.isDigit(stack.peek())){
                    k = (stack.pop() - '0') * base + k;
                    base *= 10;
                }
                //put back the substring in stack k times
                while(k-- > 0){
                    for(int i=sb.length()-1; i>=0; i--){
                        stack.push(sb.charAt(i));
                    }
                }
            }
        }
        char[] result = new char[stack.size()];
        for(int i=stack.size()-1;i>=0;i--)
            result[i] = stack.pop();
        return new String(result);


        
    }
}