Rotate a square grid of numbers 90 degrees clockwise without using extra space.
Walk around a matrix in a spiral from the outside in, collecting elements.
If any cell in a grid is zero, zero out its entire row and column.
Keep replacing a number with the sum of the squares of its digits. It's happy if you eventually reach 1.
Compute x to the power n efficiently, handling negative exponents too.
Multiply two large numbers given as strings without using built-in big integer conversion.