Finding two absent numbers between 1 and 100

You receive $98$ different integers, all between $1$ and $100$, so exactly two values are missing. How can you identify the two missing numbers efficiently?

Use the sum and the sum of squares in one $O(n)$ pass. If $x,y$ are missing, $x+y=5050-\sum a_i$ and $x^2+y^2=338350-\sum a_i^2$. Then $xy=\tfrac12\left((x+y)^2-(x^2+y^2)\right)$, so $x,y$ are the roots of a quadratic. A boolean array of size $100$ also works in $O(n)$ time with $O(n)$ space.