Mini Parser

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.
   

Solution

/**
 * // 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();
 * }
 */
public class Solution {
   public NestedInteger deserialize(String s) {
        Stack<NestedInteger> nestedIntegerStack = new Stack<>();
        int start = -1;
        boolean flag = true;

        NestedInteger cur = null;
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (ch == '[') {

                if (cur != null) {
                    nestedIntegerStack.push(cur);
                }

                cur = new NestedInteger();
                start = -1;
                flag = true;

            } else if (ch == ']') {
                String num = "";
                if (start != -1) {
                    num = s.substring(start, i);
                    start = -1;
                }


                if (!num.isEmpty()) {
                    cur.add(new NestedInteger(Integer.valueOf(num)));
                }

                if (!nestedIntegerStack.isEmpty()) {
                    NestedInteger pop = nestedIntegerStack.pop();
                    pop.add(cur);
                    cur = pop;
                }

                flag = true;
            } else if (ch == ',') {
                if (start != -1) {
                    cur.add(new NestedInteger(Integer.valueOf(s.substring(start, i))));
                    start = -1;
                }
                flag = true;
            } else {
                if (flag) {
                    start = i;
                    flag = false;
                }
            }
        }

        if (start != -1) {
            return new NestedInteger(Integer.valueOf(s.substring(start)));
        }

        return cur;
    }
}