Add to Array-Form of Integer LeetCode solution in C++

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Time : O(max(n, Log k)) Space : O(1)
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                                    class Solution {
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 public:
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  vector<int> addToArrayForm(vector<int> &num, int k) {
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    vector<int> res;
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    int i = size(num) - 1;
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    int c = 0, sum;
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    while (i >= 0) {
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      sum = num[i] + k % 10 + c;
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      res.push_back(sum % 10);
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      c = sum / 10;
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      k /= 10;
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      i--;
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    }
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    while (k) {
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      sum = k % 10 + c;
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      res.push_back(sum % 10);
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      c = sum / 10;
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      k /= 10;
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    }
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    if (c) res.push_back(c);
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    reverse(begin(res), end(res));
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    return res;
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  }
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};
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--> Approach 2
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    class Solution {
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 public:
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  vector<int> addToArrayForm(vector<int> &num, int k) {
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    vector<int> res;
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    int i = size(num) - 1;
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    int sum;
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    while (i >= 0) {
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      sum = num[i] + k;
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      res.push_back(sum % 10);
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      k = sum / 10;
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      i--;
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    }
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    while (k) {
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      res.push_back(k % 10);
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      k /= 10;
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    }
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    reverse(begin(res), end(res));
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    return res;
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  }
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};