Merge Sorted Array

Two Pointers

15 min

Problem Statement

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.

Example

Example 1:

Input: nums1 = [1,2,3,0,0,0], m = 3, nums2 = [2,5,6], n = 3

nums1: [1, 2, 3, 0, 0, 0], m: 3, nums2: [2, 5, 6], n: 3

Output: [1, 2, 2, 3, 5, 6]

Output: [1,2,2,3,5,6]

Solution

The two-pointer approach is efficient here. We start from the end of both arrays and fill `nums1` from the end. This avoids overwriting elements in `nums1` that we haven't processed yet.

Algorithm Steps

Initialize three pointers: `p1` to `m-1`, `p2` to `n-1`, and `p` to `m+n-1`.
While `p1` and `p2` are non-negative:
Compare `nums1[p1]` and `nums2[p2]`.
Place the larger element at `nums1[p]` and decrement the corresponding pointer and `p`.
If there are remaining elements in `nums2`, copy them to the beginning of `nums1`.

Merge Sorted Array

Two Pointers Approach

Input: nums1 = [1, 2, 3, 0, 0, 0], m = 3, nums2 = [2, 5, 6], n = 3

Output: [1, 2, 3, 0, 0, 0]

Progress1 / 1

Ready to start the visualization

nums1

nums2

Merge Sorted Array Solution


class Solution:
    def merge(self, nums1: list[int], m: int, nums2: list[int], n: int) -> None:
        """
        Do not return anything, modify nums1 in-place instead.
        """
        p1 = m - 1
        p2 = n - 1
        p = m + n - 1
        
        while p1 >= 0 and p2 >= 0:
            if nums1[p1] > nums2[p2]:
                nums1[p] = nums1[p1]
                p1 -= 1
            else:
                nums1[p] = nums2[p2]
                p2 -= 1
            p -= 1
            
        while p2 >= 0:
            nums1[p] = nums2[p2]
            p2 -= 1
            p -= 1