I am kinda new to CC. Can someone please explain how I can get AC in one subtask while WA in all ? Thanks!
That subtask is some special case for which your code is working
For all other cases , your code fails 
To get AC in a Subtask, you need to get AC for each Task that is part of that Subtask i.e. in order get AC in Subtask 1, you need to get AC for all of Tasks 1, 2 and 4, which you don’t seem to have done, I’m afraid i.e. you didn’t complete any Subtasks 
Edit:
I wrote a bit more about Subtasks and Constraints here which may or may not be useful to you 
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I just learned that my code was checked against 6 input files. Correct submission for file number (1,2 and 4) is necessary for the Sub-task 1 and (0,3 and 5) for Sub-task 3. I got it right?
I have been trying on this code since afternoon. Does only one correct subtask mean that I must be close ? Hoping so 
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Seems right to me 
Fingers crossed