# LeetCode 66. Plus One

## Description

https://leetcode.com/problems/plus-one/

Given a non-empty array of digits representing a non-negative integer, plus one to the integer.

The digits are stored such that the most significant digit is at the head of the list, and each element in the array contain a single digit.

You may assume the integer does not contain any leading zero, except the number 0 itself.

Example 1:

```Input: [1,2,3]
Output: [1,2,4]
Explanation: The array represents the integer 123.
```

Example 2:

```Input: [4,3,2,1]
Output: [4,3,2,2]
Explanation: The array represents the integer 4321.```

## Explanation

Use sum and carry to track the effect when the given digits plus one.

## Python Solution

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

carry = 0
i = len(digits) - 1

while (i >= 0):
sum = digits[i] + carry
if i == len(digits) - 1:
sum = sum + 1

digit = sum % 10
carry = sum // 10

result.insert(0, digit)
i = i - 1

if carry > 0:
result.insert(0, carry)

return result``````
• Time complexity: O(N)
• Space complexity: O(N)