[LeetCode] 543. Diameter of Binary Tree

Given a binary tree, you need to compute 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.
Example:
Given a binary tree 
          1
         / \
        2   3
       / \     
      4   5
Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].
Note: The length of path between two nodes is represented by the number of edges between them.

Thought process:
Length of the longest path that includes root = max depth of root.left + max depth of root.right. Use an instance variable to keep track of longest diameter as I traverse the tree.

Solution:
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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int diameter = 0;
    
    public int diameterOfBinaryTree(TreeNode root) {
        getHeight(root);
        return diameter;
    }
    
    private int getHeight(TreeNode root) {
        if (root == null) {
            return 0;
        }
        
        int left = getHeight(root.left);
        int right = getHeight(root.right);
        diameter = Math.max(diameter, left + right);
        return Math.max(left, right) + 1;
    }
}

Time complexity: O(n).
Location: Philadelphia, PA, USA

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