added comments

src/problems/lec4.rs

@@ -14,19 +14,28 @@ pub fn max_path_sum(root: Option<Rc<RefCell<TreeNode>>>) -> i32 {
 }
 
 pub fn max_sum_helper(root: &Option<Rc<RefCell<TreeNode>>>, acc: &mut i32) -> i32 {
+
     match root {
         None => 0,
         Some(val) => {
             let root_val = val.as_ref().borrow();
 
+            // max left path sum
             let left_sum = max_sum_helper(&root_val.left, acc).max(0);
+
+            // max right path sum
             let right_sum = max_sum_helper(&root_val.right, acc).max(0);
 
+            // sum of the paths
             let mut total_sum = root_val.val + left_sum + right_sum;
 
-            // shall I consider the total sum in the path sum
+            // I have to take into account the total sum of root, left child and right child,
+            // as it is possible that this sum is greater than the max path sum of going 
+            // deep donw the tree
             *acc = *acc.max(&mut total_sum); 
 
+            // this is the result I want to return from a subtree
+            // note that this does not depend on acc
             root_val.val + left_sum.max(right_sum)
         }
     }