算法Sedgewick第四版-第1章基础-1.4 Analysis of Algorithms-006BitonicMax

 package algorithms.analysis14;

 import algorithms.util.StdOut;
 import algorithms.util.StdRandom;

 /******************************************************************************
  *  Compilation:  javac BitonicMax.java
  *  Execution:    java BitonicMax N
  *  Dependencies: StdOut.java
  *
  *  Find the maximum in a bitonic array (strictly increasing, then strictly
  *  decreasing) of size N in log N time.
  *
  *  % java BitonicMax N
  *
  ******************************************************************************/

 public class BitonicMax {

     // create a bitonic array of size N
     public static int[] bitonic(int N) {
         int mid = StdRandom.uniform(N);
         int[] a = new int[N];
         for (int i = 1; i < mid; i++) {
             a[i] = a[i-1] + 1 + StdRandom.uniform(9);
         }

         if (mid > 0) a[mid] = a[mid-1] + StdRandom.uniform(10) - 5;

         for (int i = mid + 1; i < N; i++) {
             a[i] = a[i-1] - 1 - StdRandom.uniform(9);
         }

         for (int i = 0; i < N; i++) {
             StdOut.println(a[i]);
         }
         return a;
     } 

     // find the index of the maximum in a bitonic subarray a[lo..hi]
     public static int max(int[] a, int lo, int hi) {
         if (hi == lo) return hi;
         int mid = lo + (hi - lo) / 2;
         if (a[mid] < a[mid + 1]) return max(a, mid+1, hi);
         if (a[mid] > a[mid + 1]) return max(a, lo, mid);
         else return mid;
     } 

     public static void main(String[] args) {

         int N = Integer.parseInt(args[0]);
         int[] a = bitonic(N);
         StdOut.println("max = " + a[max(a, 0, N-1)]);
     }
 }