leetcode104_二叉树最大深度

public int maxDepth(TreeNode root) {
    int ans = 0;
    if(root == null) return 0;
    Deque<TreeNode> q = new LinkedList();
    q.offer(root);
    while(!q.isEmpty()) {
        int size = q.size();
        ans++;
        for(int i = 0; i < size; i++) {
            TreeNode node = q.poll();
            if(node.left != null) q.offer(node.left);
            if(node.right != null) q.offer(node.right);
        }
    }
    return ans;
}