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.

Solution:

The max length of a node is the max length of its left subtree + the max length of its right subtree.

We traverse all nodes and maintains a global max.



Code:

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