The Euclidean traveling-salesman problem is the problem of determining the shortest closed tour that connects a given set of n points in the plane. Figure 15.9(a) shows the solution to a 7- point problem. The general problem is NP-complete, and its solution is therefore believed to require more than polynomial time (see Chapter 34).

Figure 15.9: Seven points in the plane, shown on a unit grid. (a) The shortest closed tour, with length approximately 24.89. This tour is not bitonic. (b) The shortest bitonic tour for the same set of points. Its length is approximately 25.58. J. L. Bentley has suggested that we simplify the problem by restricting our attention to bitonic tours, that is, tours that start at the leftmost point, go strictly left to right to the rightmost point, and then go strictly right to left back to the starting point. Figure 15.9(b) shows the shortest bitonic tour of the same 7 points. In this case, a polynomial-time algorithm is possible.
Describe an O (n2)-time algorithm for determining an optimal bitonic tour. You may assume that no two points have the same x-coordinate.

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