Description:

Given two sorted integer arrays nums1 and nums2, merge nums2 into nums1 as one sorted array.

Note:

The number of elements initialized in nums1 and nums2 are m and n respectively.
You may assume that nums1 has enough space (size that is greater or equal to m + n) to hold additional elements from nums2.

Example:

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

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

Solution:

import heapq
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.
        """
        heapq.heapify(nums2)
        for i in range(m):
            heapq.heappush(nums2,nums1[i])
            nums1[i] = heapq.heappop(nums2)
        for i in range(m,m+n):
            nums1[i] = heapq.heappop(nums2)

Runtime: 36 ms, faster than 84.22% of Python3 online submissions for Merge Sorted Array.
Memory Usage: 13.2 MB, less than 40.49% of Python3 online submissions for Merge Sorted Array.

import heapq
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.
        """
        heapq.heapify(nums2)
        for i in range(m):
            heapq.heappush(nums2,nums1[i])
            nums1[i] = heapq.heappop(nums2)
        nums1[m:m+n] = sorted(nums2) # changed

NOTE: slower than the first version
Runtime: 60 ms, faster than 11.01% of Python3 online submissions for Merge Sorted Array.
Memory Usage: 13.2 MB, less than 46.71% of Python3 online submissions for Merge Sorted Array.

sample 16 ms submission:

NOTE: 从右往左对比"假·nums1"和"nums2"，覆盖"真·nums1"的最右

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