LeetCode 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.

题意为判断一颗二叉树是不是对称的。

解法是递归，一颗二叉树对称，必须是其左右孩子互相对称，左孩子的左子树对称于右孩子的右子树，左孩子的右子树对称于右孩子的左子树。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
*     TreeNode(int x) { val = x; }     
 * }
 */
public class Solution {
    
    private boolean isChildSymmetric(TreeNode root1,TreeNode root2) {
        if (root1==null && root2==null) {
            return true;
        }
        if (root1==null || root2==null) {
            return false;
        }
        if (root1.val==root2.val) {
            if (isChildSymmetric(root1.left, root2.right) && isChildSymmetric(root1.right, root2.left)) {
                return true;
            } else {
                return false;
            }
        } else {
            return false;
        }
    }
    
    
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        return isChildSymmetric(root.left, root.right);
    }
}