Find missing number with duplicates:

Count inversions in a array using merge sort

• http://www.geeksforgeeks.org/counting-inversions/

Two elements whose sum is closest to zero

• http://www.geeksforgeeks.org/two-elements-whose-sum-is-closest-to-zero/ Ceiling in a sorted array
• http://www.geeksforgeeks.org/ceiling-in-a-sorted-array/