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hard
Given two slices of sorted arrays, find the median. Repeat for many such slices, return the median of the results.
Task Score
100%
Correctness
Not assessed
Performance
Not assessed
Task description

The median of a sequence of numbers X[0], X[1], ..., X[N] is the middle element in terms of their values. More formally, the median of X[0], X[1], ..., X[N] is an element X[I] of the sequence, such that at most half of the elements are larger than X[I] and at most half of the elements are smaller than X[I]. For example, the median of the following sequence:

X[0] = 7 X[1] = 2 X[2] = 5 X[3] = 2 X[4] = 8

is 5; the median of the following sequence:

X[0] = 2 X[1] = 2 X[2] = 5 X[3] = 2

is 2; and the following sequence:

X[0] = 1 X[1] = 5 X[2] = 7 X[3] = 4 X[4] = 2 X[5] = 8

has two medians: 4 and 5.

Note that sequences of odd length have only one median, which is equal to X[N/2] after sorting X. In this problem we consider medians of sequences of odd length only.

Write a function:

def solution(A, B, P, Q, R, S)

that, given:

  • two non-empty arrays, A (consisting of N integers) and B (consisting of M integers), both sorted in ascending order
  • two arrays P and Q, each consisting of K indices of array A, such that 0 ≤ P[I] ≤ Q[I] < N for 0 ≤ I < K
  • two arrays R and S, each consisting of K indices of array B, such that 0 ≤ R[I] ≤ S[I] < M for 0 ≤ I < K

computes medians of K sequences of the form:

A[P[I]], A[P[I]+1], ..., A[Q[I]-1], A[Q[I]], B[R[I]], B[R[I]+1], ..., B[S[I]-1], B[S[I]]

for 0 ≤ I < K, and returns the median of all such medians.

For example, given the following arrays:

A[0] = -2 A[1] = 4 A[2] = 10 A[3] = 13 B[0] = 5 B[1] = 6 B[2] = 8 B[3] = 12 B[4] = 13 P[0] = 2 P[1] = 1 P[2] = 0 Q[0] = 3 Q[1] = 2 Q[2] = 3 R[0] = 0 R[1] = 0 R[2] = 1 S[0] = 4 S[1] = 0 S[2] = 3

the function should return 8, since:

  • the median of [10, 13, 5, 6, 8, 12, 13] equals 10,
  • the median of [4, 10, 5] equals 5,
  • the median of [−2, 4, 10, 13, 6, 8, 12] equals 8, and
  • the median of [10, 5, 8] equals 8.

Write an efficient algorithm for the following assumptions:

  • N and M are integers within the range [1..100,000];
  • K is an integer within the range [1..10,000];
  • each element of arrays A, B is an integer within the range [−1,000,000,000..1,000,000,000];
  • array A is sorted in non-decreasing order;
  • array B is sorted in non-decreasing order;
  • each element of arrays P, Q is an integer within the range [0..N−1];
  • each element of arrays R, S is an integer within the range [0..M−1];
  • P[i] ≤ Q[i] and R[i] ≤ S[i] for 0 ≤ i < K;
  • K is odd and so is Q[i]−P[i]+R[i]−S[i] for 0 ≤ i < K.
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Solution
Programming language used Python
Total time used 1 minutes
Effective time used 1 minutes
Notes
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