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Pascal's triangle starts with a single 1 at the top. Every row after that begins and ends with 1, and each number in the middle is the sum of the two numbers just above it. Row 0 is 1. Row 1 is 1, 1. Row 2 is 1, 2, 1. Row 3 is 1, 3, 3, 1.

A small hint for this one. You don't need the whole triangle. To make a new row, you only ever need the row just before it, so you can keep just one row and replace it each time.