Untitled

// Returns the length of the longest path (number of nodes) under the root
// that have the value same as the root. The path could either be
// on the left or right child of the root. The length includes the root as well.
class Solution {
    int ans;

    int solve(TreeNode root, int parent) {
        if (root == null) {
            return 0;
        }

        //The longest univalue path will cover nodes on both sides of the root.
        int left = solve(root.left, root.val);
        int right = solve(root.right, root.val);

        //The longest univalue path will cover nodes on both sides of the root.
        ans = Math.max(ans, left + right);

        // The number of nodes will be zero if the root value isn't equal to the root.
        // Otherwise return the max of left and right nodes plus one for the root itself.
        return root.val == parent ? Math.max(left, right) + 1 : 0;
    }

    public int longestUnivaluePath(TreeNode root) {
        // Use -1 for the parent value for the tree root node.
        solve(root, -1);

        return ans;
    }
}