Charging Lithium Ion batteries 18650
Generally when dealing with lithium based batteries you need to be extra careful and you have to know exactly what are you doing, because there is a great risk of fire or explosion when you handle them unproperly (overcharging, short circuit, overtemperature etc.).
If you want to charge multiple cells in series, you should always use balancing circuit that takes care of charging all cells to the exactly same voltage. When you do not use a balancer, the cells will eventually get charged to different voltages after few cycles. The difference in voltages between each cell will get bigger over time and if you are lucky, you will only end up with a dead cell in a battery pack; if you are unlucky, it may catch fire.
Also, discharging an unbalanced pack will lower it's overall capacity and may even lead to  negative voltage on some of the cells.

I looked into the datasheet and the charging voltage is 4.2V with a current of 1A.


connect it to a 42V 2A charger

It is OK to charge the cells with lower current than their maximum rating, but you should never exceed the maximum charging current, that they are specified at. (Charging them with lower currents will actually prolong their life)
There are many articles on battery balancing on the internet, you should go through them first.