[leetcode]Symmetric Tree

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

Note:
Bonus points if you could solve it both recursively and iteratively.

算法思路：

思路：递归。镜像判断。

代码如下：

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