Given a list of $n$ cities $C$ and a symmetric distance function $d: C \times C \rightarrow \mathbb{R}$. What is an ordering on $C$, $c_i \in C$ with $1 \leq i \leq n$ that visits every city and has minimum length $d(c_n, c_1) + \sum_{i=1}^{n-1} d(c_i, c_{i+1})$?