Lists, Indexing & Slicing for CP

In competitive Python, the list is your go-to array. It stores items in order and grows as needed, so most problems start here.

Lists use zero-based indexing, so a[0] is the first item. Forgetting this is the classic off-by-one bug that costs easy points.

Negative indices count from the right, so a[-1] is the last element. No need to compute len(a) - 1 yourself.

Use len(a) to get the element count. Valid indices then run from 0 up to len(a) - 1, never further.

A slice a[i:j] takes items from index i up to but not including j. The right end is always excluded.

Leave a side blank to reach the edge. a[:k] takes the first k items and a[k:] takes everything from k onward.

A third number is the step. a[::2] takes every second element, handy for splitting into even and odd positions.

A step of -1 flips the list. a[::-1] gives a reversed copy without touching the original, great for palindromes.

Slicing returns a fresh list, so a[:] is a quick shallow copy. Editing the copy will not change the original array.

Reading a[len(a)] raises an IndexError, an instant runtime error verdict. Always check that an index sits inside the list.

Unlike indexing, an out-of-range slice is forgiving. a[2:99] just returns whatever exists, so it quietly clips to the end.

You can now index from both ends, slice ranges, step, and reverse safely. These list basics are the foundation for every array problem ahead. 🚀