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.

Solution: Big Integer

Process from right to left (lest significant to most signification). Use carry to indicate whether need +1 for next digit or not.

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

class Solution {
public:
  vector<int> plusOne(vector<int>& digits) {
    int carry = 1;
    for (int i = digits.size() - 1; i >= 0; --i) {
      digits[i] += carry;
      carry = digits[i] / 10;
      digits[i] %= 10;
    }
    if (carry) digits.insert(begin(digits), carry);
    return digits;
  }
};

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