# Symmetric Tree

Given the root of a binary tree, *check whether it is a mirror of itself* (i.e., symmetric around its center).

```
def isSymmetric(self, root):
def isSym(L,R):
if not L and not R: return True
if L and R and L.val == R.val:
return isSym(L.left, R.right) and isSym(L.right, R.left)
return False
return isSym(root, root)
```

## Related Problems

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists into one **sorted** list. The list should be made by splicing together the nodes of the first two lists.

Return *the head of the merged linked list*.

Given the root of a binary tree, flatten the tree into a "linked list":

The "linked list" should use the same TreeNode class where the right child pointer points to the next node in the list and the left child pointer is always null.

The "linked list" should be in the same order as a pre-order traversal of the binary tree.

Given the root of a binary tree, return *the length of the longest path, where each node in the path has the same value*. This path may or may not pass through the root.

**The length of the path** between two nodes is represented by the number of edges between them.

Given the root of a binary tree and an integer targetSum, return true if the tree has a **root-to-leaf** path such that adding up all the values along the path equals targetSum.

A **leaf** is a node with no children.