03.求给定二叉树的最大深度， 最大深度是指树的根结点到最远叶子结点的最长路径上结点的数量。

解法1：递归

public static int maxDepth(TreeNode root) {
        // write code here
        if (root == null) {
            return 0;
        }
        return 1+Math.max(maxDepth(root.left),maxDepth(root.right));
    }

解法二：队列

public static class TreeNode {
        int val = 0;
        TreeNode left = null;
        TreeNode right = null;

        public TreeNode(int val) {
            this.val = val;
        }
    }

    public static void main(String[] args) {
        TreeNode treeNode = new TreeNode(3);
        TreeNode treeNode2 = new TreeNode(9);
        TreeNode treeNode3 = new TreeNode(20);
        TreeNode treeNode4 = new TreeNode(15);
        TreeNode treeNode5 = new TreeNode(7);
        treeNode.left = treeNode2;
        treeNode.right = treeNode3;
        treeNode2.left = treeNode4;
        treeNode2.right = treeNode5;
        System.out.println(maxDepth(treeNode));
    }

    /*
     * 使用队列来是实现层序遍历
     */
    public static int maxDepth(TreeNode root) {
        // write code here
        if (root == null) {
            return 0;
        }
        int count = 0;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int elementSize = queue.size();
            for (int i = 0; i < elementSize; i++) {
                if (queue.element().left != null) {
                    queue.offer(queue.element().left);
                }
                if (queue.element().right != null) {
                    queue.offer(queue.element().right);
                }
                queue.poll();
            }
            count++;
        }
        return count;
    }