剑指Offer-题55（一）（Java版）：二叉树的深度

参考自：《剑指Offer——名企面试官精讲典型编程题》

题目：二叉树的深度
输入一棵二叉树的根结点，求该树的深度。从根结点到叶结点依次经过的结点（含根、叶结点）形成树的一条路径，最长路径的长度为树的深度。

主要思路：使用递归，分别计算当前节点的左右子树的深度，取两者最大值，再加1。或者借助队列，使用层序遍历，每遍历一层，深度加1.

关键点：递归，层序遍历

时间复杂度：O(n)

public class TreeDepth
{
    public static void main(String[] args)
    {
//            10
//         /      \
//        6        14
//       /\        /\
//      4  8     12  16
        TreeNode root = TreeNode.generateBinaryTree();
        System.out.println(computeDepth(root)); //4
        System.out.println(computeDepthByLayer(root)); //4
    }

    //递归计算树深度
    private static int computeDepth(TreeNode root)
    {
        if (root == null) return 0;
        int leftDepth = computeDepth(root.left);
        int rightDepth = computeDepth(root.right);
        int result = leftDepth + 1;
        if (rightDepth > leftDepth)
        {
            result = rightDepth + 1;
        }
        return result;
    }

    //按层遍历计算树深度
    private static int computeDepthByLayer(TreeNode root)
    {
        if (root == null) return 0;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int depth = 0;
        while (queue.size() != 0)
        {
            depth++;
            //当前层的节点数
            int size = queue.size();
            //遍历当前层
            for (int i = 0; i < size; i++)
            {
                TreeNode top = queue.poll();
                if (top.left != null)
                {
                    queue.add(top.left);
                }
                if (top.right != null)
                {
                    queue.add(top.right);
                }
            }
        }
        return depth;
    }
}

class TreeNode
{
    public  int val;
    public  TreeNode left;
    public  TreeNode right;

    public TreeNode(int x)
    {
        val = x;
    }

//            10
//         /      \
//        6        14
//       /\        /\
//      4  8     12  16
    /**
     * 生成二叉搜索树
     * @return
     */
    public static TreeNode generateBinaryTree()
    {
        TreeNode root = new TreeNode(10);
        TreeNode node6 = new TreeNode(6);
        TreeNode node14 = new TreeNode(14);
        TreeNode node4 = new TreeNode(4);
        TreeNode node8 = new TreeNode(8);
        TreeNode node12 = new TreeNode(12);
        TreeNode node16 = new TreeNode(16);
        connectNode(root, node6, node14);
        connectNode(node6, node4, node8);
        connectNode(node14, node12, node16);
        return root;
    }

    public static void connectNode(TreeNode root, TreeNode left, TreeNode right)
    {
        root.left = left;
        root.right = right;
    }
}