LeetCode(101)——Symmetric Tree

題目：

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

 

和100題類似，只不過比較子節點時，要拿左子節點和右子節點比較

AC:

    private boolean helper(TreeNode left, TreeNode right) {
        if (left == null && right == null) {
            return true;
        }
        
        if (left == null || right == null) {
            return false;
        }
        
        return left.val == right.val && helper(left.right, right.left) && helper(left.left, right.right);
        
    }
    
    public boolean isSymmetric(TreeNode root) {
        if (root == null || (root.left == null && root.right == null)) {
            return true;
        }
        
        return helper(root.left, root.right);
    }