LeetCode 88 Merge Sorted Array

Description

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 nums2 into nums1 as one sorted array in-place.

The final sorted array should also be sorted in non-decreasing order.

Note: 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.

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Example 1

Input: nums1 = [1,2,3,0,0,0], m = 3, nums2 = [2,5,6], n = 3
Output: [1,2,2,3,5,6]
Explanation: The arrays we are merging are [1,2,3] and [2,5,6].
The result of the merge is [1,2,2,3,5,6] with the underlined elements coming from nums1.

Example 2

Input: nums1 = [1], m = 1, nums2 = [], n = 0
Output: [1]

Example 3

Input: nums1 = [0], m = 0, nums2 = [1], n = 1
Output: [1]
Explanation: The arrays we are merging are [] and [1].
The result of the merge is [1].
Note that because m = 0, there are no elements in nums1. The 0 is just a placeholder.

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Constraints

• nums1.length == m + n
• nums2.length == n
• 0 <= m, n <= 200
• 1 <= nums1[i], nums2[j] <= 10⁹
• nums1 and nums2 are sorted in non-decreasing order

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Follow up

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

Solution

Merge from the end to avoid overwriting elements in nums1

var merge = function (nums1, m, nums2, n) {
  let idx1 = m - 1
  let idx2 = n - 1
  let idx = m + n - 1
  while (idx1 >= 0 && idx2 >= 0) {
    if (nums1[idx1] > nums2[idx2]) {
      nums1[idx--] = nums1[idx1--]
    } else {
      nums1[idx--] = nums2[idx2--]
    }
  }
  while (idx2 >= 0) {
    nums1[idx--] = nums2[idx2--]
  }
};

Time Complexity: O(m + n)

Space Complexity: O(1)