動機

從背後來就是easy,從正面來就是地獄

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.

 

Example 1:

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

Example 3:

Input: nums1 = [0], m = 0, nums2 = [1], n = 1Output: [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 only there to ensure the merge result can fit in nums1.

 

Constraints:

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

 

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

Sol

class Solution:
    def merge(self, ns1: List[int], m: int, ns2: List[int], n: int) -> None:
        i, a, b = m+n-1, m-1, n-1
        while i >= 0:
            if b >= 0 and a >= 0:
                if ns2[b] > ns1[a]:
                    ns1[i], i = ns2[b], i-1
                    b -= 1
                else:
                    ns1[i], i = ns1[a], i-1
                    a -= 1
            elif b >= 0:
                ns1[i], i = ns2[b], i-1
                b -= 1
            else:
                ns1[i], i = ns1[a], i-1
                a -= 1