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/**

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

* <p>

* 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

* <p>

* Note:

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

* <p>

* Created by drfish on 28/05/2017.

*/

public class _101SymmetricTree {

public boolean isSymmetric(TreeNode root) {

if (root == null) {

return true;

}

return check(root.left, root.right);

}

private boolean check(TreeNode p, TreeNode q) {

if (p == null && q == null) {

return true;

}

if (p != null && q != null) {

if (p.val == q.val) {

return check(p.left, q.right) && check(p.right, q.left);

}

}

return false;

}

}