88 Merge Sorted Array

Description

You are given two sorted arrays of integers: nums1 and nums2.

The first array, nums1, has enough space to hold both arrays combined.

Your task is to merge nums2 into nums1 in sorted order, in-place.

Initially, nums1 contains m valid elements, followed by enough zeros to hold n elements from nums2.

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

class Solution {
    public void merge(int[] nums1, int m, int[] nums2, int n) {
        var idx1 = m - 1;
        var idx2 = n - 1;
        var 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)