Adding polynomials

Suppose you'd like to add two polynomials: $a_n x^n$ $+$ $\cdots$ $+$ $a_1 x$ $+$ $a_0$, and $b_m x^m$ $+$ $\cdots$ $+$ $b_1 x$ $+$ $b_0$. Their sum has degree $k$ $=$ $\max(m,n)$:

$$c^k x^k + \cdots + c_1 x + c_0$$

...where $c_i$ $=$ $a_i+b_i$ ($a_i$ is zero if $i>n$, and $b_i$ is zero if $i>m$).

Implement the function plus specified in Polynomial.h. You need to check that none of your coefficients fall outside $\pm$INT_MAX, specified in <limits.h> (in which case you should return {NULL, OVERFLOW}). For a polynomial of degree $n$, your function should have complexity $O(n)$.