Fills element in arr [ ] from its pair sum array pair [ ] . n is size of arr [ ]
void constructArr ( int arr [ ] , int pair [ ] , int n ) { arr [ 0 ] = ( pair [ 0 ] + pair [ 1 ] - pair [ n - 1 ] ) / 2 ; for ( int i = 1 ; i < n ; i ++ ) arr [ i ] = pair [ i - 1 ] - arr [ 0 ] ; }