DecodedStringAtIndex.java

package stack;

import java.util.Stack;

/**
 * Created by gouthamvidyapradhan on 12/05/2019
 */
public class DecodedStringAtIndex {

  public static void main(String[] args) {
    System.out.println(new DecodedStringAtIndex().decodeAtIndex("ha22", 5));
  }

    class Node{
        String S;
        long count;
        int multiple;
        Node(String S, long count, int multiple){
            this.S = S;
            this.count = count;
            this.multiple = multiple;
        }
    }

    public String decodeAtIndex(String S, int K) {
        Stack<Node> stack = new Stack<>();
        StringBuilder sb = new StringBuilder();
        char prev = ' ';
        for(char c : S.toCharArray()){
            if(Character.isDigit(c)){
                String currStr = sb.toString();
                long len = 0L;
                if(!stack.isEmpty()){
                    len = stack.peek().count * stack.peek().multiple;
                }
                stack.push(new Node(currStr, len + (currStr.length()),  Integer.parseInt(String.valueOf(c))));
                if(((len + (currStr.length())) * Integer.parseInt(String.valueOf(c))) >= K){
                    break;
                }
                prev = c;
            } else{
                if(Character.isDigit(prev)){
                    sb = new StringBuilder();
                }
                sb.append(c);
                prev = c;
            }
        }
        while(!stack.isEmpty()){
            Node top = stack.peek();
            long l = top.count;
            if(K <= l){
                return String.valueOf(top.S.charAt((int)l - K - 1));
            }
            long mod = (K % l);
            if(mod == 0){
                return String.valueOf(top.S.charAt(top.S.length() - 1));
            }
            if(l - top.S.length() < mod){
                long i = l - mod;
                return String.valueOf(top.S.charAt(top.S.length() - (int)i - 1));
            } else{
                stack.pop();
            }
        }
        return "";
    }
}