Trace ["wrt","wrf","er","ett","rftt"].
Compare adjacent pairs:
- "wrt" vs "wrf": differ at index 2. 't' < 'f'.
- "wrf" vs "er": differ at index 0. 'w' < 'e'.
- "er" vs "ett": differ at index 1. 'r' < 't'.
- "ett" vs "rftt": differ at index 0. 'e' < 'r'.
Graph edges: t→f, w→e, r→t, e→r.
In-degrees: w:0, e:1, r:1, t:1, f:1.
Topological sort starting with 'w': w → e → r → t → f.
Result: "wertf".
where is total characters across all words. Building graph and toposort both linear in unique characters and edges.