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

递归解决，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 symmetric(TreeNode l,TreeNode r) {
        if(l==null&&r==null) return true;
        if(l==null||r==null) return false;
        if(l.val!=r.val) return false;
        if(!(symmetric(l.left,r.right)&&symmetric(l.right,r.left))) return false;
        return true;
    }
    
    public boolean isSymmetric(TreeNode root) {
        if(root==null) return true;
        return symmetric(root.left,root.right);
    }
}