101. Symmetric Tree

class Solution {
public boolean isSymmetric(TreeNode root) {
if (root == null){
return true;
}

Queue<TreeNode> queue = new LinkedList<>();
queue.add(root.left);
queue.add(root.right);

while(!queue.isEmpty()){
TreeNode left = queue.remove();
TreeNode right = queue.remove();

if (left == null && right == null){
continue;
}
if (left == null || right == null){
return false;
}
if (left.val != right.val){
return false;
}

queue.add(left.left);
queue.add(right.right);
queue.add(left.right);
queue.add(right.left);
}
return true;
}

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

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