Flatten Nested List Iterator
<p>You are given a nested list of integers <code>nestedList</code>. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.</p>

<p>Implement the <code>NestedIterator</code> class:</p>

<ul>
	<li><code>NestedIterator(List&lt;NestedInteger&gt; nestedList)</code> Initializes the iterator with the nested list <code>nestedList</code>.</li>
	<li><code>int next()</code> Returns the next integer in the nested list.</li>
	<li><code>boolean hasNext()</code> Returns <code>true</code> if there are still some integers in the nested list and <code>false</code> otherwise.</li>
</ul>

<p>Your code will be tested with the following pseudocode:</p>

<pre>
initialize iterator with nestedList
res = []
while iterator.hasNext()
    append iterator.next() to the end of res
return res
</pre>

<p>If <code>res</code> matches the expected flattened list, then your code will be judged as correct.</p>

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

<pre>
<strong>Input:</strong> nestedList = [[1,1],2,[1,1]]
<strong>Output:</strong> [1,1,2,1,1]
<strong>Explanation:</strong> By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].
</pre>

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

<pre>
<strong>Input:</strong> nestedList = [1,[4,[6]]]
<strong>Output:</strong> [1,4,6]
<strong>Explanation:</strong> By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].
</pre>

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

<ul>
	<li><code>1 &lt;= nestedList.length &lt;= 500</code></li>
	<li>The values of the integers in the nested list is in the range <code>[-10<sup>6</sup>, 10<sup>6</sup>]</code>.</li>
</ul>
Stack; Tree; Depth-First Search; Design; Queue; Iterator
/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds, if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Return the nested list that this NestedInteger holds, if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */

class NestedIterator {
public:
    NestedIterator(vector<NestedInteger>& nestedList) {
        auto dfs = [&](this auto&& dfs, vector<NestedInteger>& ls) -> void {
            for (auto& x : ls) {
                if (x.isInteger()) {
                    nums.push_back(x.getInteger());
                } else {
                    dfs(x.getList());
                }
            }
        };
        dfs(nestedList);
    }

    int next() {
        return nums[++i];
    }

    bool hasNext() {
        return i + 1 < nums.size();
    }

private:
    vector<int> nums;
    int i = -1;
};

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i(nestedList);
 * while (i.hasNext()) cout << i.next();
 */
