1. Description

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

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

2. Example

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

3. Code

public class LeetCode0101 {

     public class TreeNode {
          int val;
          TreeNode left;
          TreeNode right;
          TreeNode(int x) { val = x; }
      }

    public boolean isSymmetric(TreeNode root) {
        if(root == null){
            return true;
        }
        return isSymmetricTree(root.left,root.right);
    }
    private boolean isSymmetricTree(TreeNode p, TreeNode q){
        if(p==null && q==null) {
            return true;
        }
        if(p==null && q!=null||p!=null && q==null) {
            return false;
        }
        if(p.val!= q.val) {
            return false;
        }
        return isSymmetricTree(p.left,q.right) && isSymmetricTree(p.right, q.left);
    }
}