https://leetcode.com/problems/mini-parser/

/**
 * // 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.equals("")) {
            return new NestedInteger();
        }
        if (s.startsWith("[")) {
            return impl(s.substring(1, s.length()-1));
        }
        return new NestedInteger(Integer.parseInt(s));
    }

    private NestedInteger impl(String s) {
        if (s.equals("")) {
            return new NestedInteger();
        }
        char [] chList = s.toCharArray();
        int wrap = 0;
        int left = 0;
        NestedInteger ret = new NestedInteger();
        for (int i = 0; i < chList.length; i++) {
            if (chList[i] == '[') {
                wrap++;
            }
            else if (chList[i] == ']') {
                wrap--;
            }
            else if (chList[i] == ',' && wrap == 0) {
                ret.add(deserialize(s.substring(left, i)));
                left = i + 1;
            }
        }
        ret.add(deserialize(s.substring(left, chList.length)));
        return ret;
    }
}