10. merge sort

DIVIDE & CONQUER

time complexity- O(NlogN)

space complexity- O(N)

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For Splitting:-

1. We pass low and high to the function
2. find middle
3. recurse for ( low to mid ) & ( mid+1 to high )
4. call merge function for these low, mid, high
5. done







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For merging:-
we give the function: low, mid, high
and break the array into intervals : (left to mid), (mid+1 to high) 
and then use 2 pointer
----------------------
temp goes from i=low to i=high, so at last, we store numbers in array using i-low to make it 0 based index

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