题目描述：
Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
For example, this binary tree is symmetric:
1
/ \
2 2
/ \ / \
3 4 4 3
But the following is not:
1
/ \
2 2
\ \
3 3
判断一个树的左右子树是否完全相等。
思路：分别对左右子树进行中序遍历，然后对遍历结果进行一一对比。
实现代码：
public class Solution {
public bool IsSymmetric(TreeNode root) {
if(root == null || root.left ==null && root.right == null){
return true;
}
var left = new List<string>();
var right = new List<string>();
TravelL(root.left, left);
TravelR(root.right, right);
if(left.Count != right.Count){
return false;
}
for(var i = 0;i < left.Count; i++){
if(left[i] != right[i]){
return false;
}
}
return true;
}
public void TravelL(TreeNode node , IList<string> result){
if(node == null){
result.Add("N");
return;
}
result.Add(node.val.ToString());
TravelL(node.left, result);
TravelL(node.right, result);
}
public void TravelR(TreeNode node , IList<string> result){
if(node == null){
result.Add("N");
return;
}
result.Add(node.val.ToString());
TravelR(node.right, result);
TravelR(node.left, result);
}
}