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
 * type NestedInteger struct {
 * }
 *
 * // Return true if this NestedInteger holds a single integer, rather than a nested list.
 * func (this NestedInteger) IsInteger() bool {}
 *
 * // 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
 * // So before calling this method, you should have a check
 * func (this NestedInteger) GetInteger() int {}
 *
 * // Set this NestedInteger to hold a single integer.
 * func (n *NestedInteger) SetInteger(value int) {}
 *
 * // Set this NestedInteger to hold a nested list and adds a nested integer to it.
 * func (this *NestedInteger) Add(elem NestedInteger) {}
 *
 * // Return the nested list that this NestedInteger holds, if it holds a nested list
 * // The list length is zero if this NestedInteger holds a single integer
 * // You can access NestedInteger's List element directly if you want to modify it
 * func (this NestedInteger) GetList() []*NestedInteger {}
 */

type NestedIterator struct {
	nums []int
	i    int
}

func Constructor(nestedList []*NestedInteger) *NestedIterator {
	var dfs func([]*NestedInteger)
	nums := []int{}
	i := -1
	dfs = func(ls []*NestedInteger) {
		for _, x := range ls {
			if x.IsInteger() {
				nums = append(nums, x.GetInteger())
			} else {
				dfs(x.GetList())
			}
		}
	}
	dfs(nestedList)
	return &NestedIterator{nums, i}
}

func (this *NestedIterator) Next() int {
	this.i++
	return this.nums[this.i]
}

func (this *NestedIterator) HasNext() bool {
	return this.i+1 < len(this.nums)
}