LeetCode Q101: Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

Solution 1: Recursive solution.
Similar to question of check whether two trees are identical. Here, the difference is the order we access the left and right sub-trees of two trees.



Solution 2: Iterative solution.