Leetcode 101. Symmetric Tree

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

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.

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public class Solution {
public boolean isSymmetric(TreeNode root) {
if(root == null){
return true;
}
if(symmetric(root.left,root.right)){
return true;
}
else{
return false;
}
}
private boolean symmetric(TreeNode left, TreeNode right){
if(left == null && right == null ){
return true;
}
if(left == null || right == null){
return false;
}
if(left.val != right.val){
return false;
}
return (symmetric(left.left,right.right))&&(symmetric(left.right,right.left));
}
}