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

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

**Intuition.** The number is stored most-significant digit first. Adding one only affects the end — unless you hit a chain of 9s. Walk from the **last digit**: if it's less than 9, increment and you're done; if it's 9, set it to 0 and carry left. If the carry runs off the front (all 9s), prepend a 1. [1,2,9] + 1: 9→0 carry, 2→3 stop → [1,3,0] 1 <rect x="180" y="40" width="56" height="40" rx="5"

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Explain the plus-one carry.

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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]

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