# LeetCode 66. Plus One

## Description

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

You are given a large integer represented as an integer array `digits`, where each `digits[i]` is the `ith` digit of the integer. The digits are ordered from most significant to least significant in left-to-right order. The large integer does not contain any leading `0`‘s.

Increment the large integer by one and return the resulting array of digits.

Example 1:

```Input: digits = [1,2,3]
Output: [1,2,4]
Explanation: The array represents the integer 123.
Incrementing by one gives 123 + 1 = 124.
Thus, the result should be [1,2,4].
```

Example 2:

```Input: digits = [4,3,2,1]
Output: [4,3,2,2]
Explanation: The array represents the integer 4321.
Incrementing by one gives 4321 + 1 = 4322.
Thus, the result should be [4,3,2,2].
```

Example 3:

```Input: digits = 
Output: 
Explanation: The array represents the integer 0.
Incrementing by one gives 0 + 1 = 1.
Thus, the result should be .
```

Example 4:

```Input: digits = 
Output: [1,0]
Explanation: The array represents the integer 9.
Incrementing by one gives 9 + 1 = 10.
Thus, the result should be [1,0].
```

Constraints:

• `1 <= digits.length <= 100`
• `0 <= digits[i] <= 9`
• `digits` does not contain any leading `0`‘s.

## Explanation

Use sum and carry to track the effect when the given digits plusing by 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)