Formatted question description: https://leetcode.ca/all/1073.html

1073. Adding Two Negabinary Numbers (Medium)

Given two numbers arr1 and arr2 in base -2, return the result of adding them together.

Each number is given in array format:  as an array of 0s and 1s, from most significant bit to least significant bit.  For example, arr = [1,1,0,1] represents the number (-2)^3 + (-2)^2 + (-2)^0 = -3.  A number arr in array format is also guaranteed to have no leading zeros: either arr == [0] or arr[0] == 1.

Return the result of adding arr1 and arr2 in the same format: as an array of 0s and 1s with no leading zeros.

 

Example 1:

Input: arr1 = [1,1,1,1,1], arr2 = [1,0,1]
Output: [1,0,0,0,0]
Explanation: arr1 represents 11, arr2 represents 5, the output represents 16.

 

Note:

  1. 1 <= arr1.length <= 1000
  2. 1 <= arr2.length <= 1000
  3. arr1 and arr2 have no leading zeros
  4. arr1[i] is 0 or 1
  5. arr2[i] is 0 or 1

Solution 1.

When adding two numbers in base 2 I use a variable carry to store the carryover.

Assume we are adding two bits a, b and carry. The sum can be 0, 1, 2, 3, and the leftover bit value is sum % 2 and the new value of carry is sum / 2.

Let’s try the same approach for this problem.

Assume the current bit represents k = (-2)^n, (n >= 0).

  • Consider carry = 0, sum can be 0, 1, 2.
    • When sum = 0 or 1, leftover = sum, carry = 0.
    • When sum = 2, the value is 2k = -1 * (-2k), so it’s the same as leftover = 1, carry = -1
  • Consider carry = -1, sum can be -1, 0, 1.
    • For the new case sum = -1, the value is -k = -2k + k, so it’s the same as leftover = 1, carry = 1.
  • Consider carry = 1, sum can be 1, 2, 3.
    • For the new case sum = 3, the value is 3k = -1 * (-2k) + k, so it’s the same as leftover = 1, carry = -1.

Now we’ve considerred all cases. In sum:

sum leftover carry
-1 1 1
0 0 0
1 1 0
2 0 -1
3 1 -1

The pattern is:

leftover = (sum + 2) % 2
carry = 1 - (sum + 2) / 2

Or

leftover = sum & 1
carry = -(sum >> 1)
// OJ: https://leetcode.com/problems/adding-two-negabinary-numbers/

// Time: O(N)
// Space: O(N)
class Solution {
public:
    vector<int> addNegabinary(vector<int>& A, vector<int>& B) {
        vector<int> ans;
        for (int i = A.size() - 1, j = B.size() - 1, carry = 0; i >= 0 || j >= 0 || carry;) {
            if (i >= 0) carry += A[i--];
            if (j >= 0) carry += B[j--];
            ans.push_back(carry & 1);
            carry = -(carry >> 1);
        }
        while (ans.size() > 1 && ans.back() == 0) ans.pop_back();
        reverse(ans.begin(), ans.end());
        return ans;
    }
};

Java

class Solution {
    public int[] addNegabinary(int[] arr1, int[] arr2) {
        int length1 = arr1.length, length2 = arr2.length;
        int sumLength = Math.max(length1, length2) + 2;
        int[] arr1Ext = new int[sumLength];
        System.arraycopy(arr1, 0, arr1Ext, sumLength - length1, length1);
        int[] arr2Ext = new int[sumLength];
        System.arraycopy(arr2, 0, arr2Ext, sumLength - length2, length2);
        int[] sumArr = new int[sumLength];
        for (int i = sumLength - 1; i > 0; i--) {
            sumArr[i] += arr1Ext[i] + arr2Ext[i];
            int carry = sumArr[i] >> 1;
            sumArr[i] -= carry * 2;
            sumArr[i - 1] -= carry;
        }
        int startIndex = sumLength - 1;
        for (int i = 0; i < sumLength; i++) {
            if (sumArr[i] != 0) {
                startIndex = i;
                break;
            }
        }
        int trimLength = sumLength - startIndex;
        int[] sumArrTrim = new int[trimLength];
        System.arraycopy(sumArr, startIndex, sumArrTrim, 0, trimLength);
        return sumArrTrim;
    }
}

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