101. Symmetric Tree

Problem:

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

Analysis:
如果当前对比的node1 和node 2相等，要符合symmetric tree我们需要比较node1.left == node2.right && node1.right == node2.left.
Solution:

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