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easyDSA

How do you add one to a number represented as a digit array?

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01

Understand the problem

Explain the plus-one carry.

arraysmath
02

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Nudge consolestandby

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03

Study the solution

Walk from the last digit: if it's < 9, increment and return; if it's 9, set it to 0 and carry to the left. If you carry past the front (all 9s), prepend a 1. O(n) in the worst case, O(1) usually.

Solution ready — 2 min read

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04

Read the code

Carry from the last digit
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def plus_one(digits):
    for i in range(len(digits) - 1, -1, -1):
        if digits[i] < 9:
            digits[i] += 1
            return digits
        digits[i] = 0
    return [1] + digits        # all nines


# --- demo ---
print(plus_one([1, 2, 9]))   # [1, 3, 0]
print(plus_one([9, 9, 9]))   # [1, 0, 0, 0]
05

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