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    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

    Solution:

    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    public class Solution {
        public boolean isSymmetric(TreeNode root) {        
            if(root==null) return true;
            return isMirror(root.left,root.right);
        }
            public boolean isMirror(TreeNode p, TreeNode q) {
                if(p==null && q==null) return true;
                if(p==null || q==null) return false;
                return (p.val==q.val) && isMirror(p.left,q.right) && isMirror(p.right,q.left);
            }
        }
    }

     

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