Post-apocalyptic novel with smart teenage girl who survived in a bomb shelter built by her father. She thinks she's the only one left, but isn't. Probably from the 1970s. I liked it then, curious what I'd think now.
Emergence, by David Palmer?
It fits each of the points you provide:
- The main character is teenage girl (actually 11yo, but close enough)
- She has a super high IQ
- Survives in a bomb shelter under her home
- Ends up finding other people who are left
It's not from the 1970's though (1984)
Possibly Z for Zachariah by Robert O'Brian. "Nuclear war has devastated America, except for the valley where Ann Burden has been living alone for over a year - until Loomis, a scientist in a radiation-proof suit, arrives." Published in 1976.