Day44_LeetCode394.java

class Solution {
    public String decodeString(String s) {
        Stack<Integer> numberStack = new Stack<>();
        Stack<String> mainStack = new Stack<>();

        for(int i=0;i<s.length();i++){
            char ch = s.charAt(i);

            if(ch>='0' && ch<='9'){ // identifing it is a number
                int num = 0 ;

                while(i<s.length() && s.charAt(i)>='0' && s.charAt(i)<='9'){
                    num = num * 10  + (int)(s.charAt(i)-'0');
                    i++;
                }
                i--; // to maintain the loop variable
                numberStack.push(num);
            }else if ( ch != ']'){
                mainStack.push(ch + "");
            }else { // condition for ']'
                String str = "";
                while(!mainStack.peek().equals("[")){
                    str = mainStack.pop() + str ;
                }
                mainStack.pop();
                int repetationNumber = numberStack.pop();

                StringBuilder sb = new StringBuilder("") ;

                while(repetationNumber>0){
                    sb.append(str);
                    repetationNumber--;
                }

                 mainStack.push(sb.toString());
            }
        }

        StringBuilder ans = new StringBuilder("");

        while(mainStack.size()>0){
            ans.insert(0,mainStack.pop());
        }
        return ans.toString();
    }
}