Sword finger offer 28. symmetric binary tree

Title Description

Problem analysis

Ontology uses queues to do , First, judge whether the subtree is empty
Then put left and right , Determine whether the element of the queue is empty , Four conditions must be considered .
Then put the left side of the left subtree The right side of the right subtree is put into the queue in turn （ Put in pairs ）

Code instance

class Solution {
    
    public boolean isSymmetric(TreeNode root) {
    
        Queue<TreeNode> deque = new LinkedList<>();
        if(root == null){
    
            return true;
        }
        deque.offer(root.left);
        deque.offer(root.right);
        while (!deque.isEmpty()) {
    
            TreeNode leftNode = deque.poll();
            TreeNode rightNode = deque.poll();
            if (leftNode == null && rightNode == null) {
    
                continue;
            }// If all are empty, judge , Space symmetry continues 
// if (leftNode == null && rightNode != null) {
    
// return false;
// }
// if (leftNode != null && rightNode == null) {
    
// return false;
// }
// if (leftNode.val != rightNode.val) {
    
// return false;
// }
            //  The above three judgment conditions are combined 
            if (leftNode == null || rightNode == null || leftNode.val != rightNode.val) {
    
                return false;
            }
            //  Here is the sequence and use Deque Different 
            deque.offer(leftNode.left);
            deque.offer(rightNode.right);
            deque.offer(leftNode.right);
            deque.offer(rightNode.left);
        }
        return true;
}
}