题目：https://leetcode-cn.com/problems/symmetric-tree
答案：
递归
public boolean isSymmetric(TreeNode root) {
if(root==null) return true;
return isSy(root.left,root.right);
}
public boolean isSy(TreeNode left, TreeNode right){
if(left==null){
if(right==null) {
return true;
}else {
return false;
}
}else {
if(right==null) {
return false;
}else {
return left.val == right.val && isSy(left.left,right.right) && isSy(left.right,right.left);
}
}
}