101. Symmetric Tree

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
代码如下：

 /**
  * 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;
         TreeNode left=root.left;
         TreeNode right=root.right;

         if(LeftTraverse(left).equals(RightTraverse(right)))
         return true;

         return false;
     }
     public List<String> LeftTraverse(TreeNode root){
         List<String> list=new ArrayList<>();
         if(root==null)
         return list;

         list.add(String.valueOf(root.val));
         if(root.left!=null)
         list.addAll(LeftTraverse(root.left));
         else
         list.add(" ");

         if(root.right!=null)
         list.addAll(LeftTraverse(root.right));
         else
         list.add(" ");
         return list;
     }
      public List<String> RightTraverse(TreeNode root){
         List<String> list=new ArrayList<>();
         if(root==null)
         return list;

         list.add(String.valueOf(root.val));
         if(root.right!=null)
         list.addAll(RightTraverse(root.right));
         else
         list.add(" ");

         if(root.left!=null)
         list.addAll(RightTraverse(root.left));
         else
         list.add(" ");

         return list;
     }
 }