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.

class Solution {
    public int diameterOfBinaryTree(TreeNode root) {
        int[] max = new int[1];
        helper(root, max);
        return max[0] - 1;
    }

    public int helper(TreeNode root, int[] max) {
        if (root == null) return 0;
        int leftMaxHeight = helper(root.left, max);
        int rightMaxHeight = helper(root.right, max);
        
        int tempMax = leftMaxHeight + rightMaxHeight + 1;
        max[0] = Math.max(max[0], tempMax);
        if (leftMaxHeight > rightMaxHeight) {
            return leftMaxHeight + 1;
        }
        return rightMaxHeight + 1;
    }
}