989. Add to Array-Form of Integer

1. Description

The array-form of an integer num is an array representing its digits in left to right order.

• For example, for num = 1321, the array form is [1,3,2,1].

Given num, the array-form of an integer, and an integer k, return the array-form of the integer num + k.

2. Example

Example 1:
Input: num = [1,2,0,0], k = 34
Output: [1,2,3,4]
Explanation: 1200 + 34 = 1234

Example 2:
Input: num = [2,7,4], k = 181
Output: [4,5,5]
Explanation: 274 + 181 = 455

Example 3:
Input: num = [2,1,5], k = 806
Output: [1,0,2,1]
Explanation: 215 + 806 = 1021

3. Constraints

• 1 <= num.length <= $10^4$
• 0 <= num[i] <= 9
• num does not contain any leading zeros except for the zero itself.
• 1 <= k <= $10^4$

4. Solutions

Math

n = num.size()
Time complexity: O(max(n, logk)):
Space complexity: O(1)

class Solution {
public:
    vector<int> addToArrayForm(const vector<int> &num, int k) {
        vector<int> result;
        int carry = 0;
        for (int i = num.size() - 1; i >= 0 || k > 0; --i, k /= 10) {
            int sum = (i >= 0 ? num[j] : 0) + k % 10 + carry;
            result.push_back(sum % 10);
            carry = sum / 10;
        }

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

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