DiameterOfBinaryTree.java

public class DiameterOfBinaryTree {
    /**
         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.
     */


    /** T(n) = O(n) */

    private int max = 0;
    public int diameterOfBinaryTree(TreeNode root) {
        if (root == null) return 0;
        helper(root);
        return max;
    }

    /**
     return logest path
     */
    private int helper(TreeNode node) {
        if (node == null) return 0;
        int lPath = helper(node.left);
        int rPath = helper(node.right);
        int myPath = lPath + rPath;
        max = myPath > max ? myPath : max;
        return Math.max(lPath, rPath) + 1;
    }
}