[Leetcode] Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

使用递归写比较容易，使用iterative的方法的话需要使用一个队列来进行层次遍历，使用dummy节点来表示一层的结束。注意根节点如果为null的话返回值应该是0，不是null的话至少为1而不是0.

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

    public int maxDepth(TreeNode root) {
        Queue<TreeNode> qu = new LinkedList<TreeNode>();
        TreeNode dummy = new TreeNode(0);
        int depth = 0;
        if(root == null) return depth;
        
        qu.offer(root); qu.offer(dummy); depth++;
        while(qu.size() != 0) {
            TreeNode temp = qu.poll();
            if(temp == dummy && qu.size() != 0) {
                depth++;
                qu.offer(dummy);
            }
            if(temp.left != null) qu.offer(temp.left);
            if(temp.right != null) qu.offer(temp.right);
        }
        
        return depth;
    }