September 3, 2014

Finding Kth largest element in Array

See Selection Algorithm first: http://en.wikipedia.org/wiki/Selection_algorithm
package org.udanax.eddieyoon.codeguru;

public class KthLargest {

  public static void main(String[] args) {
    int[] x = new int[] { 3, 6, 92, 34, 1, 35, 62, 13, 12, 24, 53 };
    System.out.println(getKthLargest(x, 3));
  }

  private static int getKthLargest(int[] x, int k) {
    int low = 0;
    int high = x.length - 1;

    while (true) {
      int pivot = (low + high) / 2;
      int newPiv = partition(x, low, high, pivot);

      if (newPiv == k) {
        return x[newPiv];
      } else if (newPiv < k) {
        low = newPiv + 1;
      } else {
        high = newPiv - 1;
      }
    }
  }

  private static int partition(int[] x, int left, int right, int pivot) {
    int pivValue = x[pivot];
    swap(x, pivot, right);
    int storePos = left;

    for (int i = left; i < right; i++) {
      if (x[i] < pivValue) {
        swap(x, i, storePos);
        storePos++;
      }
    }
    swap(x, storePos, right);
    return storePos;
  }

  private static void swap(int[] x, int a, int b) {
    int temp = x[a];
    x[a] = x[b];
    x[b] = temp;
  }

}

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