#APL+WIN, 21 bytes

Prompts for input of u followed by a. Outputs b immediately over the top of c with no separator:

(a←⎕)[u;u←⎕]⋄a[⍋u;⍋u]

Try it online! Courtesy of Dyalog Classic

APL+WIN, 21 bytes

Prompts for input of u followed by a. Outputs b immediately over the top of c with no separator:

(a←⎕)[u;u←⎕]⋄a[⍋u;⍋u]

Try it online! Courtesy of Dyalog Classic