假设我们有以下 df：
+---+---------+---------+--------+-------+
| | 2016 | 2017 | 2018 | 2019 |
+---+---------+---------+--------+-------+
| 0 | 26560.0 | 26810.0 | NaN | NaN |
| 1 | 570.0 | NaN | 550.0 | 540.0 |
| 2 | 3770.0 | 3450.0 | 3210.0 | NaN |
| 3 | 4320.0 | NaN | NaN | NaN |
+---+---------+---------+--------+-------+
我想添加两个额外的列“值”和“年”。在“值”列中会有最近一年的值，在“年”列中应该有最近一年没有缺失值：
+---+---------+---------+--------+-------+---------+------+
| | 2016 | 2017 | 2018 | 2019 | value | year |
+---+---------+---------+--------+-------+---------+------+
| 0 | 26560.0 …