Maximum Width of Binary Tree
<p>Given the <code>root</code> of a binary tree, return <em>the <strong>maximum width</strong> of the given tree</em>.</p>

<p>The <strong>maximum width</strong> of a tree is the maximum <strong>width</strong> among all levels.</p>

<p>The <strong>width</strong> of one level is defined as the length between the end-nodes (the leftmost and rightmost non-null nodes), where the null nodes between the end-nodes that would be present in a complete binary tree extending down to that level are also counted into the length calculation.</p>

<p>It is <strong>guaranteed</strong> that the answer will in the range of a <strong>32-bit</strong> signed integer.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0600-0699/0662.Maximum%20Width%20of%20Binary%20Tree/images/width1-tree.jpg" style="width: 359px; height: 302px;" />
<pre>
<strong>Input:</strong> root = [1,3,2,5,3,null,9]
<strong>Output:</strong> 4
<strong>Explanation:</strong> The maximum width exists in the third level with length 4 (5,3,null,9).
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0600-0699/0662.Maximum%20Width%20of%20Binary%20Tree/images/maximum-width-of-binary-tree-v3.jpg" style="width: 442px; height: 422px;" />
<pre>
<strong>Input:</strong> root = [1,3,2,5,null,null,9,6,null,7]
<strong>Output:</strong> 7
<strong>Explanation:</strong> The maximum width exists in the fourth level with length 7 (6,null,null,null,null,null,7).
</pre>

<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0600-0699/0662.Maximum%20Width%20of%20Binary%20Tree/images/width3-tree.jpg" style="width: 289px; height: 299px;" />
<pre>
<strong>Input:</strong> root = [1,3,2,5]
<strong>Output:</strong> 2
<strong>Explanation:</strong> The maximum width exists in the second level with length 2 (3,2).
</pre>

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

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 3000]</code>.</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
</ul>
Tree; Depth-First Search; Breadth-First Search; Binary Tree
/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func widthOfBinaryTree(root *TreeNode) int {
	q := []pair{{root, 1}}
	ans := 0
	for len(q) > 0 {
		ans = max(ans, q[len(q)-1].i-q[0].i+1)
		for n := len(q); n > 0; n-- {
			p := q[0]
			q = q[1:]
			root = p.node
			if root.Left != nil {
				q = append(q, pair{root.Left, p.i << 1})
			}
			if root.Right != nil {
				q = append(q, pair{root.Right, p.i<<1 | 1})
			}
		}
	}
	return ans
}

type pair struct {
	node *TreeNode
	i    int
}