
recursively
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean isSymmetric(TreeNode root) {
if(root == null){
return true;
}
else {
return Symmetric(root.left, root.right);
}
}
public boolean Symmetric(TreeNode t1, TreeNode t2) {
if(t1 == null && t2 == null) return true;
if((t1 == null) ^ (t2 == null)) return false;
if(t1.val != t2.val){
return false;
}
else {
return Symmetric(t1.left, t2.right) && Symmetric(t1.right, t2.left);
}
}
}