# LeetCode 66: Plus One — Step-by-Step Visual Trace

**Easy** — Array | Math | Simulation

## The Problem

Given a non-empty array of decimal digits representing a non-negative integer, increment the integer by one and return the resulting digits as an array.

## Approach

Iterate through the digits array from right to left (least to most significant digit). If a digit is less than 9, increment it and return immediately. If it's 9, set it to 0 and continue. If all digits were 9, prepend a 1 to handle the overflow case.

**Time:** O(n) · **Space:** O(1)

## Code

```python
class Solution:
    def plusOne(self, digits: List[int]) -> List[int]:
        n = len(digits)

        for i in range(n - 1, -1, -1):
            if digits[i] < 9:
                digits[i] += 1
                return digits
            digits[i] = 0

        return [1] + digits
```

## Watch It Run

> **[Open interactive visualization](https://tracelit.dev/app?trace=0066_plus-one)**

> **Try it yourself:** Open [TraceLit](https://tracelit.dev/app?trace=0066_plus-one) and step through every line.

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