Merge two sorted arrays without using additional memory We have two sorted arrays of integers. 
Without using additional memory we need to merge these two arrays such that the smallest numbers are in the 1st array and the remaining numbers are in the second array.

For instance, if your two arrays are $A=[1,3,9,14]$ and $B=[2,4,15]$, the result should be $A=[1,2,3,4], B=[9,14,15]$. Entries are integers, and you can use only $O(1)$ extra memory for auxiliary variables.
Here, great solution, from: http://www.geeksforgeeks.org/merge-two-sorted-arrays-o1-extra-space/

Below is C++ implementation of above algorithm.

// C++ program for implementation of Sieve of Atkin
#include &lt;bits/stdc++.h&gt;
using namespace std;

// Merge ar1[] and ar2[] with O(1) extra space
void merge(int ar1[], int ar2[], int m, int n)
{
    // Iterate through all elements of ar2[] starting from
    // the last element
    for (int i=n-1; i&gt;=0; i--)
    {
        /* Find the smallest element greater than ar2[i]. Move all
           elements one position ahead till the smallest greater
           element is not found */
        int j, last = ar1[m-1];
        for (j=m-1; j &gt;= 0 &amp;&amp; ar1[j] &gt; ar2[i]; j--)
            ar1[j+1] = ar1[j];

        // If there was a greater element
        if (j != m-1)
        {
            ar1[j+1] = ar2[i];
            ar2[i] = last;
        }
    }
}