374.Non-Decreasing Codes
★★★combinatoricsCh. 1 — Classical Probability and Combinatorics444+ Problems in Probability
Pizza Hut gives new users a 7-digit code. Digits may repeat and codes may start with 0. How many such codes have non-decreasing digits?
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A non-decreasing string is fully determined by how many times each digit appears — stars and bars.
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