Initialization: Before the first iteration of the while the only change of the max-heap is that A[i]
is increased an may therefore violate the max-heap property.
Maintenance: Immediately before the iteration i and the child that violated the max-heap prop-
erty has been exchanged thus restoring the max-heap property between these. This can only
destroy the max-heap property between i and the parent of i.
Termination: The termination condition of the while states that at the end of the iteration the
max-heap property between i and its parent is restored or the i is the root of the heap.
We see by the loop invariant that the heap property is restored at end of the iteration.
6.5 − 7
Algorithm 5 HEAP-DELETE(A, i)
Input: A max-heap A and integers i.
Output: The heap A with the element a position i deleted.
A[i] ↔A[heap-size[A]]
heap-size[A] ←heap-size[A] − 1
key ←A[i]
if key 6 A[PARENT(i)] then
MAX-HEAPIFY(A, i)
else
while i > 1 and A[PARENT(i)] < key do
A[i] ↔A[PARENT(i)]
i ←PARENT(i)
end while
end if
6.5 − 8
Given k sorted lists with a total of n elements show how to merge them in O(n lg k) time. Insert
all k elements a position 1 from each list into a heap. Use EXTRACT-MAX to obtain the first element
of the merged list. Insert element at position 2 from the list where the largest element originally
came from into the heap. Continuing in this fashion yields the desired algorithm. Clearly the
running time is O(n lg k).
6 − 2
a. A d-ary heap can be implemented using a dimensional array as follows. The root is kept in
A[1], its d children are kept in order in A[2] through A[d+1] and so on. The procedures to map a
node with index i to its parent and its jth child are given by:
D-ARY-PARENT(i)
return b(i − 2)/d + 1c
D-ARY-CHILD(i, j)
return d(i − 1) + j + 1
b. Since each node has d children the height of the tree is Θ(log
d
n).
c. The HEAP-EXTRACT-MAX algorithm given in the text works fine for d-ary heaps; the problem
is MAX-HEAPIFY. Here we need to compare the argument node to all its children. This takes
Θ(d log
d
n) and dominates the overall time spent by HEAP-EXTRACT-MAX.
9
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