decode-string.java

// Problem link - https://leetcode.com/problems/decode-string/

class Solution {
    public String decodeString(String s) {
        Stack<Character> stack = new Stack<>();

        for(char c: s.toCharArray()){
            if(c != ']'){
                stack.push(c);
            } else {
                StringBuilder sb = new StringBuilder();
                while(!stack.isEmpty() && Character.isLetter(stack.peek())){
                    sb.insert(0, stack.pop());
                }
                String sub = sb.toString();
                stack.pop();

                sb = new StringBuilder();
                while(!stack.isEmpty() && Character.isDigit(stack.peek())){
                    sb.insert(0, stack.pop());
                }
                int count = Integer.valueOf(sb.toString());

                while(count > 0){
                    for(char ch: sub.toCharArray()){
                        stack.push(ch);
                    }
                    count--;
                }
            }
        }

        StringBuilder ans = new StringBuilder();
        while(!stack.isEmpty())
            ans.insert(0, stack.pop());

        return ans.toString();
    }
}