101. Symmetric Tree

https://leetcode.com/problems/symmetric-tree/

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        // 其實是分成兩個樹去比較(左子樹, 右子樹
        return isSymmetricSubtree(root.left, root.right);
    }
    
    private boolean isSymmetricSubtree(TreeNode left, TreeNode right) {
        if (left == null && right == null) {
            return true;
        }
        if (left == null || right == null) {
            return false;
        }
        return left.val == right.val && isSymmetricSubtree(left.left, right.right) && isSymmetricSubtree(left.right, right.left);
    }
}