Merge Sorted Array

Problem

You are given two integer arrays nums1 and nums2, sorted in non-decreasing order, and two integers m and n, representing the number of elements in nums1 and nums2 respectively.

Merge nums1 and nums2 into a single array sorted in non-decreasing order.

The final sorted array should not be returned by the function, but instead be stored inside the array nums1. To accommodate this, nums1 has a length of m + n, where the first m elements denote the elements that should be merged, and the last n elements are set to 0 and should be ignored. nums2 has a length of n.

Follow up: Can you come up with an algorithm that runs in O(m + n) time?

Constraints

• nums1.length == m + n
• nums2.length == n
• 0 <= m, n <= 200
• 1 <= m + n <= 200
• -109 <= nums1[i], nums2[j] <= 109

Solution

The problem Merge Sorted Array can be solved by keeping track of two pointers for each arrays. In order to remove the need for using vector.insert, which has time complexity linear to the elements moved, I’ll be merging the elements from the biggest to the smallest.

Implementation

class Solution
{
  public:
    void merge(vector<int> &nums1, int m, vector<int> &nums2, int n)
    {
        int idx = m + n - 1;
        int i = m - 1, j = n - 1;

        while (i >= 0 && j >= 0)
            nums1[idx--] = nums1[i] > nums2[j] ? nums1[i--] : nums2[j--];

        while (j >= 0)
            nums1[idx--] = nums2[j--];
    }
};