Update _687.java (#25)

change dfs function name to meaningful name, calculateLongestUnivaluePathFromRootToLeaves, and also additional calculation of max univalue path cross the root node should be specified somehow, one or two comments should be very helpful.

Author: jianminchen

src/main/java/com/fishercoder/solutions/_687.java

@@ -47,7 +47,9 @@ public int longestUnivaluePath(TreeNode root) {
             }
             return result[0];
         }
-
+        
+        // calculate longest univalue path from root to leaves
+        // In addition, the maximum univalue path cross the root node is calculated and then global maximum is udpated.
         private int dfs(TreeNode root, int[] result) {
             int leftPath = root.left == null ? 0 : dfs(root.left, result);
             int rightPath = root.right == null ? 0 : dfs(root.right, result);