hello,
there is only one difference in the above solutions is that at the 20th line, in the for loop’s termination condition, I have just applied some modifications and miracle, my solution accepted, can anyone tell how this happens internally?
vector<ll>::size_type is unsigned, so if it has to represent -1 - as it would have to if v were empty - it (i.e. v.size() - 1) would be represented as a gigantically huge positive number, and i would go out of the bounds of v.
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makes sense, thanks dude.
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This is why it is always a good practice to cast v.size() to int like int(v.size()) to avoid undefined behaviour. 
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certainly