# 543. Diameter of Binary Tree

## Problem

Given the root of a binary tree, return the length of the diameter of the tree.

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

The length of a path between two nodes is represented by the number of edges between them.

Example 1:

```
Input: root = [1,2,3,4,5]
Output: 3
Explanation: 3is the length of the path [4,2,1,3] or [5,2,1,3].
```

Example 2:

```
Input: root = [1,2]
Output: 1
```

Constraints:

- The number of nodes in the tree is in the range [1, 10^4].
- -100 <= Node.val <= 100

## Code

```
class Solution {
int max = 0;
public int diameterOfBinaryTree(TreeNode root) {
helper(root);
return max;
}
private int helper(TreeNode root) {
if (root == null) return 0;
int left = helper(root.left);
int right = helper(root.right);
max = Math.max(max, left + right);
return Math.max(left, right) + 1;
}
}
```

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