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JavaScript (Node.js), 53 bytes f=(a,b=[],c=a.map(([a,...c])=>(b+=[a],c)))=>b&&b+f(c) Try it online!
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# [JavaScript (Node.js)], 53 bytes <!-- language-all: lang-javascript --> f=(a,b=[],c=a.map(([a,...c])=>(b+=[a],c)))=>b&&b+f(c) [Try it online!][TIO-khtxn9kw] [JavaScript (Node.js)]: https://nodejs.org [TIO-khtxn9kw]: https://tio.run/##dY3BCsIwEETvfoWnJqExP1AiiAoeBD14Cz1s0qSNpklpa9Gvr4sepTCHGebt7B0mGEzvu3ETU2Xn2UkKXEtVciNBtNBRqoALIUzJ5JbqXCrAjjFMOst07qhhc2FSHFKwIqSaOqrIw8dEODE@2hbQtGnylpSMFas/1uBjROoU3AJx8tijbo3tl1aOry5AhNGnuEAc3oB2vbuecWr/DKOffjgnl9400Fffw/kD "JavaScript (Node.js) – Try It Online"