[LeetCode]Symmetric Tree

Top-down, 尾递归。

代码如下：

	/**
	* Definition for binary tree
	* 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 isSym(root.left, root.right);
	}
	private boolean isSym(TreeNode left, TreeNode right) {
	if (left == null && right == null)
	return true;
	else if (left != null && right != null) {
	if (left.val == right.val)
	return isSym(left.left, right.right) && isSym(left.right, right.left);
	else
	return false;
	}
	else
	return false;
	}
	}

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