Breaking_Benjamin's blog

Lucky Numbers
By Breaking_Benjamin, 10 years ago, In English

Consider Lucky numbers to be numbers with only digits 4 or 7 . Repetition is allowed. We need to generate all lucky number below 1010 in ascending order. What is the algorithm to do it ? Please explain it .

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10 years ago, # |
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brute-force

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    10 years ago, # ^ |
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    Not able to understand it. A c-code will be helpful or an algorithm.

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      10 years ago, # ^ |
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      From each lucky number X we can generate two lucky numbers — 10 * X + 4 and 10 * X + 7.

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    10 years ago, # ^ |
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    i think ur lines 58 and 65 (both having lucky.insert(nval);) are not needed because of ur line 54 (lucky.insert(val);).
    alternatively, u could remove lines 53 and 54, and leave the rest of ur program as is.

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10 years ago, # |
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You can find it with recursive.
There's at most 21 + 22 + 23...210 = 2046 lucky numbers.
Here's my code

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7 years ago, # |
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Here's a cleaner, easier and faster solution simply using strings, if anybody comes back to look at this thread

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