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PassingCars (Lesson 5)

Instructions​

A non-empty array A consisting of N integers is given. The consecutive elements of array A represent consecutive cars on a road.

Array A contains only 0s and/or 1s:

  • 0 represents a car traveling east,
  • 1 represents a car traveling west.

The goal is to count passing cars. We say that a pair of cars (P, Q), where 0 ≤ P < Q < N, is passing when P is traveling to the east and Q is traveling to the west.

For example, consider array A such that:

A[0] = 0 A[1] = 1 A[2] = 0 A[3] = 1 A[4] = 1 We have five pairs of passing cars: (0, 1), (0, 3), (0, 4), (2, 3), (2, 4).

Write a function:

function solution(A);

that, given a non-empty array A of N integers, returns the number of pairs of passing cars.

The function should return −1 if the number of pairs of passing cars exceeds 1,000,000,000.

For example, given:

A[0] = 0 A[1] = 1 A[2] = 0 A[3] = 1 A[4] = 1 the function should return 5, as explained above.

Write an efficient algorithm for the following assumptions:

  • N is an integer within the range [1..100,000];
  • each element of array A is an integer that can have one of the following values: 0, 1.

Solution​

function solution(A) {
  const totalCarLength = A.length;

  let answer = 0;
  let accZeroCount = 0;

  for (let i = 0; i < totalCarLength; i++) {
    if (A[i] === 0) {
      answer += totalCarLength - 1 - i - accZeroCount;
      accZeroCount += 1;
    }
  }

  return 1000000000 < answer ? -1 : answer;
}

Review​

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References​