Selection Algorithm - Kth Largest / Smallest from unsorted list in O(n)

SelectionAlgorithm.cpp

int A[MAX], N;

int partitions(int low, int high) {
  int p=low, r=high, x=A[r], i=p-1;
  for (int j=p; j<=r-1; j++) {
    if (A[j]<=x){
      i=i+1;
      swap(A[i],A[j]);
    }
  }
  swap(A[i+1],A[r]);
  return i+1;
}

int selection_algorithm(int left, int right, int kth) {
  for( ; ; ) {
    ////Select the Pivot Between Left and Right 
    int pivotIndex = partitions(left, right);          
    int len = pivotIndex - left + 1;
    if (kth == len)
      return A[pivotIndex];
    else if (kth < len)
      right=pivotIndex-1;
    else {
      kth=kth-len;
      left=pivotIndex+1;
    }
  }
}