Mini Parser
<p>Given a string s represents the serialization of a nested list, implement a parser to deserialize it and return <em>the deserialized</em> <code>NestedInteger</code>.</p>

<p>Each element is either an integer or a list whose elements may also be integers or other lists.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;324&quot;
<strong>Output:</strong> 324
<strong>Explanation:</strong> You should return a NestedInteger object which contains a single integer 324.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;[123,[456,[789]]]&quot;
<strong>Output:</strong> [123,[456,[789]]]
<strong>Explanation:</strong> 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
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 5 * 10<sup>4</sup></code></li>
	<li><code>s</code> consists of digits, square brackets <code>&quot;[]&quot;</code>, negative sign <code>&#39;-&#39;</code>, and commas <code>&#39;,&#39;</code>.</li>
	<li><code>s</code> is the serialization of valid <code>NestedInteger</code>.</li>
	<li>All the values in the input are in the range <code>[-10<sup>6</sup>, 10<sup>6</sup>]</code>.</li>
</ul>
Stack; Depth-First Search; String
/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *     If value is provided, then it holds a single integer
 *     Otherwise it holds an empty nested list
 *     constructor(value?: number) {
 *         ...
 *     };
 *
 *     Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     isInteger(): boolean {
 *         ...
 *     };
 *
 *     Return the single integer that this NestedInteger holds, if it holds a single integer
 *     Return null if this NestedInteger holds a nested list
 *     getInteger(): number | null {
 *         ...
 *     };
 *
 *     Set this NestedInteger to hold a single integer equal to value.
 *     setInteger(value: number) {
 *         ...
 *     };
 *
 *     Set this NestedInteger to hold a nested list and adds a nested integer elem to it.
 *     add(elem: NestedInteger) {
 *         ...
 *     };
 *
 *     Return the nested list that this NestedInteger holds,
 *     or an empty list if this NestedInteger holds a single integer
 *     getList(): NestedInteger[] {
 *         ...
 *     };
 * };
 */

function deserialize(s: string): NestedInteger {
    if (s === '' || s === '[]') {
        return new NestedInteger();
    }
    if (s[0] !== '[') {
        return new NestedInteger(+s);
    }
    const ans: NestedInteger = new NestedInteger();
    let depth = 0;
    for (let i = 1, j = 1; i < s.length; ++i) {
        if (depth === 0 && (s[i] === ',' || i === s.length - 1)) {
            ans.add(deserialize(s.slice(j, i)));
            j = i + 1;
        } else if (s[i] === '[') {
            ++depth;
        } else if (s[i] === ']') {
            --depth;
        }
    }
    return ans;
}
