題目說明:給你兩棵樹，要你判斷兩棵樹是否對稱

Case 1

Input: root = [1,2,2,3,4,4,3]
Output: true

Case 2

Input: root = [1,2,2,null,3,null,3]
Output: false

解題思路：要判斷兩種情況

1. 是否為空值(null)
2. 左邊節點數值是否=右邊節點數值

附上程式碼以及註解

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
            return symmetric(root.left,root.right);
    }
    public boolean symmetric(TreeNode n1,TreeNode n2){
        if(n1==null&&n2==null)return true;
        if(n1==null||n2==null)return false;
        if(n1.val==n2.val){
            return symmetric(n1.left,n2.right)&&symmetric(n1.right,n2.left);
        }
        return false;
    }
}