二叉树的遍历：
    力扣226，101，222，110
    1.二叉树的反转226 

    1. 递归法：
    2. if(root==null){
    3. return root;
    4. }
    5. reverseChildren(root);
    6. invertTree(root.left);
    7. invertTree(root.right);
    8. return root;
    9. }
    10. private void reverseChildren(TreeNode root){
    11.   TreeNode temp= root.right;
    12.   root.right=root.left;
    13.   root.left=temp;
    14. }
    15. 迭代法
    16. if(root==null){
    17. return true;
    18. }
    19. Deque<TreeNode>queue = new LinkedList<>();
    20. queue.addFirst(root.left);
    21. queue.addLast(root.right);
    22. while (!queue.isEmpty()){
    23.   TreeNode node1 = queue.pollFirst();
    24.   TreeNode node2 = queue.pollLast();
    25.   if(node1==null&&node2==null){
    26.     continue;
    27.   }
    28.   if(node1==null||node2==null||node1.val!=node2.val){
    29.     return false;
    30.   }
    31.   queue.addFirst(node1.right);
    32.   queue.addLast(node2.left);
    33.   queue.addFirst(node1.left);
    34.   queue.addLast(node2.right);
    35. }
    36. return true;

    2.对称二叉树101

    1. 递归法
    2. if(root==null){
    3. return true;
    4. }
    5. return mirrorTree(root.left,root.right);
    6. }
    7. private boolean mirrorTree(TreeNode left,TreeNode right){
    8.   if(left!=null&&right==null){
    9.     return false;
    10.   }else if(left==null&&right!=null){
    11.     return false;
    12.   }else if(left==null&&right==null){
    13.     return true;
    14.   }
    15.   else if(left.val!=right.val){
    16.     return false;
    17.   }
    18.   boolean outside = mirrorTree(left.left, right.right);
    19.   boolean inside = mirrorTree(left.right, right.left);
    20.     return outside&&inside;

    3.二叉树的节点个数

    1. if(root==null){
    2.   return 0;
    3. }
    4. int leftCount = countNodes(root.left);
    5. int rightCount = countNodes(root.right);
    6. return 1+leftCount+rightCount;


    4.平衡二叉树

    1. if(root==null){
    2.   return true;
    3. }
    4. getMaxDepth(root);
    5. return res;
    6. }
    7. private int getMaxDepth(TreeNode root){
    8.   if(root==null){
    9.     return 0;
    10.   }
    11.   int leftDepth = getMaxDepth(root.left);
    12.   int rightDepth = getMaxDepth(root.right);
    13.   if(Math.abs(leftDepth-rightDepth)>=2){
    14.     res=false;
    15.   }
    16.     return 1+Math.max(leftDepth,rightDepth);