<p>Given the <code>root</code> of a binary tree, return <em>the length of the <strong>diameter</strong> of the tree</em>.</p>

<p>The <strong>diameter</strong> of a binary tree is the <strong>length</strong> of the longest path between any two nodes in a tree. This path may or may not pass through the <code>root</code>.</p>

<p>The <strong>length</strong> of a path between two nodes is represented by the number of edges between them.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/03/06/diamtree.jpg" style="width: 292px; height: 302px;" />
<pre>
<strong>Input:</strong> root = [1,2,3,4,5]
<strong>Output:</strong> 3
<strong>Explanation:</strong> 3 is the length of the path [4,2,1,3] or [5,2,1,3].
</pre>

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

<pre>
<strong>Input:</strong> root = [1,2]
<strong>Output:</strong> 1
</pre>

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

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 10<sup>4</sup>]</code>.</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
</ul>

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def diameterOfBinaryTree(self, root: Optional[TreeNode]) -> int:
        stack = [root]
        depths = collections.defaultdict(int)
        last_left = False
        last_right = None
        maxlen = 0
        curlen = 0

        while stack:
            node = stack[-1]
            if node.left and not last_left:
                stack.append(node.left)
                # depths[node.left] += 1
            else:
                if node.right and last_right != node.right:
                    stack.append(node.right)
                    last_left = False
                    # depths[node.right] += 1
                else:
                    stack.pop()
                    left, right = depths.get(node.left,0), depths.get(node.right,0)
                    depths[node] = 1 + max(left,right)
                    last_right = node
                    last_left = True
            curlen = depths.get(node.left,0) + depths.get(node.right, 0)
            maxlen = max(maxlen, curlen)
        return maxlen

18300