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.

O(n) time O(k) space

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 /**
 * // 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) {
        if (s == null || s.length() == 0) {
            return null;
        }
        if (s.charAt(0) != '[') {
            return new NestedInteger(Integer.parseInt(s));
        }
        Stack<NestedInteger> stack = new Stack<>();
        NestedInteger cur = new NestedInteger();
        stack.push(cur);
        int start = 1; // 0 is [
        for (int i = 1; i < s.length(); i++) {
            if (s.charAt(i) == '[') {
                NestedInteger ni = new NestedInteger();
                stack.peek().add(ni);
                stack.push(ni);
                start = i + 1;
            } else if (s.charAt(i) == ',' || s.charAt(i) == ']') {
                if (i > start) {
                    Integer value = Integer.parseInt(s.substring(start, i));
                    stack.peek().add(new NestedInteger(value));
                }
                start = i + 1;
                if (s.charAt(i) == ']') {
                    stack.pop();
                }
            }
        }
        return cur;
    }
}