Diameter of Binary Tree

Problem Description

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.

Given a binary tree 

         / \
        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.


From problem description, longest path mah or may not pass through the root.

For example:

The idea is to calculate the longest path from left subtree, and right subtree, and record longest path (left, right, (left + right)).

Using helper class Depth to record each node depth and max diameter. This way we can optimize the time complexity to O(n).

  1. Recursively find the height of left subtree

  2. Recursively find the height of right subtree

  3. Calculate max diameter max(left diameter, right diameter, (left.height + right.height))

  4. Return max depth and max diameter.

Complexity Analysis

Time Complexity: O(n)

  • n - number of tree nodes.


class Solution {
    public int diameterOfBinaryTree(TreeNode root) {
        return getDepth(root).max;

    private Depth getDepth(TreeNode node) {
        if (node == null) return new Depth(0, 0);
        Depth left = getDepth(node.left);
        Depth right = getDepth(node.right);
        // calculate diameter of current node
        int max = Math.max(Math.max(left.max, right.max), left.depth + right.depth);
        // calculate max depth
        int depth = 1 + Math.max(left.depth, right.depth);
        return new Depth(max, depth);

    class Depth {
        int max;
        int depth;
        public Depth(int max, int depth) {
            this.max = max;
            this.depth = depth;

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