Maximum Depth of Binary Tree－求二叉树高度

非递归：
 public static int maxDepth(TreeNode root){
        if(root==null) return 0;
        Queue<TreeNode> q=new LinkedList<TreeNode>();
        q.offer(root);
        int enQueNum=1;
        int visitedNum=0;
        int lastLevNum=1; //做标记 标记到每层最后的节点 当visitedNum与它相等时本层结束
        int height=0;
        
        while(!q.isEmpty()){
            TreeNode now=q.poll();
            visitedNum++;
            if(now.left!=null){
                q.offer(now.left);
                enQueNum++;
            }
            if(now.right!=null){
                q.offer(now.right);
                enQueNum++;
            }
            if(visitedNum==lastLevNum){
                height++;
                lastLevNum=enQueNum;//lastLevNum的值赋为此时最后进队列的节点数值也是本层的最后节点
            }
        }
         return height;
     }

 递归：

public static int maxDepth2(TreeNode root){
            if(root==null) return 0;            
            int leftHeight=0;
            int rightHeight=0;
            leftHeight=maxDepth2(root.left);
            rightHeight=maxDepth2(root.right);
            return leftHeight>rightHeight?leftHeight+1:rightHeight+1;
     }