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) {
        if(s.isEmpty()) return null;
        if(s.charAt(0)!='[') return new NestedInteger(Integer.valueOf(s));
        NestedInteger cur = null;
        Stack<NestedInteger> stack = new Stack<>();
        int start = 0;
        for(int i=0; i<s.length(); i++){
            char ch = s.charAt(i);
            if(ch=='['){
                if(cur!=null) stack.push(cur);
                cur = new NestedInteger();
                start = i + 1;
            } else if(ch==','){
                if(s.charAt(i-1)!=']') cur.add(new NestedInteger(Integer.valueOf(s.substring(start, i))));
                start = i + 1;
            } else if(ch==']'){
                String str = s.substring(start, i);
                if(!str.isEmpty()) cur.add(new NestedInteger(Integer.valueOf(str)));
                if(!stack.isEmpty()){
                    NestedInteger pop = stack.pop();
                    pop.add(cur);
                    cur = pop;
                }
                start = i + 1;
            }
        }
        return cur;
    }
}