101. Symmetric Tree 二叉树是否对称

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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public bool IsSymmetric(TreeNode root) {
		if (root == null) return true;
		Queue<TreeNode> queue = new Queue<TreeNode>();
		queue.Enqueue(root);
		while (queue.Count > 0) {
			int count = queue.Count;
			List<int?> nextLevel = new List<int?>();
			for (int i = 0; i < count; i++) {
				TreeNode curNode = queue.Dequeue();
				if (curNode.left != null) {
					queue.Enqueue(curNode.left);
					nextLevel.Add(curNode.left.val);
				} else {
					nextLevel.Add(null);
				}
				if (curNode.right != null) {
					queue.Enqueue(curNode.right);
					nextLevel.Add(curNode.right.val);
				} else {
					nextLevel.Add(null);
				}
			}
			if (!LevelIsSymmetric(nextLevel)){
				return false;
			}
		}
		return true;
    }
    
	public bool LevelIsSymmetric(List<int?> list) {
		int left = 0;
		int right = list.Count - 1;
		while (right > left) {
			if (list[left++] != list[right--]) {
				return false;
			}
		}
		return true;
	}
}

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