The max depth of a binary tree

DFS

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

BFS

public int maxDepthWithQueue(TreeNode root) {
    if (root == null) {
        return 0;
    }
    Queue<TreeNode> queue = new LinkedList<TreeNode>();
    queue.offer(root);
    int depth = 0;
    while (!queue.isEmpty()) {
        /*size用来记录本层级的结点是否已经处理完*/
        int size = queue.size();
        while (size > 0) {
            TreeNode node = queue.poll();
            if (node.left != null) {
                queue.offer(node.left);
            }
            if (node.right != null) {
                queue.offer(node.right);
            }
            size--;
        }
        depth++;
    }
    return depth;

}

Time Complexity: O(n) for both