LeetCode 101. Symmetric Tree.java

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

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

But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3

Note:
Bonus points if you could solve it both recursively and iteratively.

	class Solution {
	public boolean isSymmetric(TreeNode root) {
	// corner cases
	if (root == null) {
	return true;
	}
	return isSymmetric(root.left, root.right);
	}
	public boolean isSymmetric(TreeNode left, TreeNode right) {
	// corner cases
	if (left == null && right == null) {
	return true;
	}
	if (left == null && right != null) {
	return false;
	}
	if (left != null && right == null) {
	return false;
	}
	return left.val == right.val && isSymmetric(left.left, right.right) && isSymmetric(left.right, right.left);
	}
	}

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