Decode String

class Solution {
    public String decodeString(String s) {
        
        Stack<String> sol = new Stack(); 
        Stack<Integer> multiplier = new Stack(); 
        
        String solution = ""; 
        int number = 0; 
        
        for (int i=0; i<s.length(); i++){
            if (Character.isDigit(s.charAt(i))){
                number = number * 10 + Character.getNumericValue(s.charAt(i)); 
            }else if (s.charAt(i) == '['){ 
                multiplier.push(number);
                number = 0;
                sol.push(solution);
                solution = "";
            }else if (s.charAt(i) == ']'){
                int mul = multiplier.pop(); 
                String poper = sol.pop();
                solution = poper + repeat(solution, mul);
            }else 
            {
                solution += s.charAt(i);
            }
        }
        return solution;
    }
    
    public String repeat(String toRepeat, int repeatCount) {
        StringBuilder sb = new StringBuilder();
        int i = 0;
        while (i < repeatCount) {
            sb.append(toRepeat);
            i++;
        }
        return sb.toString();
    }
}