Given a nested list of integers represented as a string, implement a parser to deserialize it.

Each element is either an integer, or a list – whose elements may also be integers or other lists.

Note: You may assume that the string is well-formed:

String is non-empty.
String does not contain white spaces.
String contains only digits 0-9, [, - ,, ].
Example 1:

Given s = "324",

You should return a NestedInteger object which contains a single integer 324.

Example 2:

Given s = "[123,[456,[789]]]",

Return a NestedInteger object containing a nested list with 2 elements:

1. An integer containing value 123.
2. A nested list containing two elements:
    i.  An integer containing value 456.
    ii. A nested list with one element:
         a. An integer containing value 789.
/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 *     // Constructor initializes an empty nested list.
 *     public NestedInteger();
 *
 *     // Constructor initializes a single integer.
 *     public NestedInteger(int value);
 *
 *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 *     public boolean isInteger();
 *
 *     // @return the single integer that this NestedInteger holds, if it holds a single integer
 *     // Return null if this NestedInteger holds a nested list
 *     public Integer getInteger();
 *
 *     // Set this NestedInteger to hold a single integer.
 *     public void setInteger(int value);
 *
 *     // Set this NestedInteger to hold a nested list and adds a nested integer to it.
 *     public void add(NestedInteger ni);
 *
 *     // @return the nested list that this NestedInteger holds, if it holds a nested list
 *     // Return null if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
class Solution
    public NestedInteger deserialize(String s) {
        Stack<NestedInteger> stack = new Stack<NestedInteger>();
        String temp = "";

        for(char c: s.toCharArray()){
            switch(c){
                case '[':
                    stack.push(new NestedInteger()); //start a new NI
                    break;

                case ']':
                    if(!temp.equals("")){
                        stack.peek().add(new NestedInteger(Integer.parseInt(temp))); //add NI to parent
                        temp="";
                    }

                    NestedInteger top = stack.pop();
                    if(!stack.empty()){
                        stack.peek().add(top);
                    }else{
                        return top;
                    }

                    break;

                case ',':
                    if(!temp.equals("")){
                        stack.peek().add(new NestedInteger(Integer.parseInt(temp)));//add NI to parent
                        temp="";
                    }

                    break;

                default:
                    temp += c;
            }
        }

        if(!temp.equals("")){
            return new NestedInteger(Integer.parseInt(temp));
        }

        return null;
    }
}