66. Plus One

1. Description

You are given a large integer represented as an integer array digits, where each digits[i] is the $i^{th}$ 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.

2. Example

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

3. Constraints

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

4. Solutions

Array

n = digits.size()

Time complexity : O(n)
Space complexity : O(1)

class Solution {
public:
    vector<int> plusOne(const vector<int> &digits) {
        const int n = digits.size();
        vector<int> result;
        result.reserve(n + 1);

        int carry = 1;
        for (int i = n - 1; i >= 0; --i) {
            int sum = digits[i] + carry;
            carry = sum / 10;
            result.push_back(sum % 10);
        }

        if (carry == 1) {
            result.push_back(1);
        }

        reverse(result.begin(), result.end());

        return result;
    }
};