Problem 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:

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

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

Solution:

Use two pointers to solve this question.

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

round 0:
     m     i
[1,2,3,0,0,0]
[2,5,6]
     n

round 1:
     m   i  
[1,2,3,0,0,6]
[2,5,6]
   n

round 2:
     m i    
[1,2,3,0,5,6]
[2,5,6]
 n      

round 3:
   m i    
[1,2,3,3,5,6]
[2,5,6]
 n      

round 4:
  mi    
[1,2,2,3,5,6]
[2,5,6]
 n

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class Solution {
public:
    void merge(vector<int>& nums1, int m, vector<int>& nums2, int n) {
        int a = m-1, b= n-1, i=m+n-1;
        
        while(a>=0 && b>=0){
            nums1[i--] = (nums1[a] > nums2[b]) ? nums1[a--] : nums2[b--];
        }
        
        while(b>=0){
            nums1[i--] = nums2[b--];
        }
    }
};

time complexity: $O(n)$
space complexity: $O(1)$
reference: