39、二叉树-对称二叉树

思路：

复制出一个二叉树b，加上原二叉树为a。

a.val==b.val

a.left.val==b.right.val

a.right.val=b.left.val

class Solution {
//递归方式
  public boolean isSymmetric02(TreeNode root) {
        if (root==null){
            return false;
        }
        return isSymmetric02(root.left,root.right);
    }

    private boolean isSymmetric02(TreeNode left, TreeNode right) {
        if (left==null||right==null){
            return left == null && right == null;
        }
        return left.val==right.val&&isSymmetric02(left.left,right.right)
                &&isSymmetric02(right.left,left.right);
    }

    public boolean isSymmetric(TreeNode root) {
        if (root==null){
            return false;
        }
        Queue<TreeNode> queue=new LinkedList<>();
        queue.add(root);
        queue.add(root);
        while (!queue.isEmpty()){
            TreeNode left = queue.poll();
            TreeNode right = queue.poll();
            if (left==null||right==null){
                if (left==null&&right==null){
                    continue;
                }else {
                    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;
    }
}

还有一种方法，先反转然后跟原来树对比是否相等