I recall that there was a Putnam problem which went something like this:
Find all real functions satisfying
$f(s^2+f(t)) = t+f(s)^2$
for all $s,t \in \mathbb{R}$.
There was a cool trick to solving it that I wanted to remember. But I don't know which test it was from and google isn't much help for searching with equations.
Does anyone know which problem I am thinking of so I can look up that trick?