JavaAlgorithms/Maths/FindMaxRecursion.java

package Maths;

import java.util.Arrays;
import java.util.Random;

public class FindMaxRecursion {
  public static void main(String[] args) {
    Random rand = new Random();

    /* rand size */
    int size = rand.nextInt(100) + 1;
    int[] array = new int[size];

    /* init array with rand numbers */
    for (int i = 0; i < size; i++) {
      array[i] = rand.nextInt() % 100;
    }

    assert max(array, array.length) == Arrays.stream(array).max().getAsInt();
    assert max(array, 0, array.length - 1) == Arrays.stream(array).max().getAsInt();
  }

  /**
   * Get max of array using divide and conquer algorithm
   *
   * @param array contains elements
   * @param low the index of the first element
   * @param high the index of the last element
   * @return max of {@code array}
   */
  public static int max(int[] array, int low, int high) {
    if (low == high) {
      return array[low]; // or array[high]
    }

    int mid = (low + high) >>> 1;

    int leftMax = max(array, low, mid); // get max in [low, mid]
    int rightMax = max(array, mid + 1, high); // get max in [mid+1, high]

    return Math.max(leftMax, rightMax);
  }

  /**
   * Get max of array using recursion algorithm
   *
   * @param array contains elements
   * @param len length of given array
   * @return max value of {@code array}
   */
  public static int max(int[] array, int len) {
    return len == 1 ? array[0] : Math.max(max(array, len - 1), array[len - 1]);
  }
}