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title	id	challengeType
Equilibrium index	5987fd532b954e0f21b5d3f6	5

Description

An equilibrium index of a sequence is an index into the sequence such that the sum of elements at lower indices is equal to the sum of elements at higher indices.

For example, in a sequence $A$:

:::: $A_0 = -7$

:::: $A_1 = 1$

:::: $A_2 = 5$

:::: $A_3 = 2$

:::: $A_4 = -4$

:::: $A_5 = 3$

:::: $A_6 = 0$

3 is an equilibrium index, because:

:::: $A_0 + A_1 + A_2 = A_4 + A_5 + A_6$

6 is also an equilibrium index, because:

:::: $A_0 + A_1 + A_2 + A_3 + A_4 + A_5 = 0$

(sum of zero elements is zero)

7 is not an equilibrium index, because it is not a valid index of sequence $A$.

Write a function that, given a sequence, returns its equilibrium indices (if any).

Assume that the sequence may be very long.

Instructions

Tests

tests:
  - text: <code>equilibrium</code> is a function.
    testString: 'assert(typeof equilibrium === "function", "<code>equilibrium</code> is a function.");'
  - text: '<code>equilibrium([-7, 1, 5, 2, -4, 3, 0])</code> should return <code>[3,6]</code>.'
    testString: 'assert.deepEqual(equilibrium(tests[0]), ans[0], "<code>equilibrium([-7, 1, 5, 2, -4, 3, 0])</code> should return <code>[3,6]</code>.");'
  - text: '<code>equilibrium([2, 4, 6])</code> should return <code>[]</code>.'
    testString: 'assert.deepEqual(equilibrium(tests[1]), ans[1], "<code>equilibrium([2, 4, 6])</code> should return <code>[]</code>.");'
  - text: '<code>equilibrium([2, 9, 2])</code> should return <code>[1]</code>.'
    testString: 'assert.deepEqual(equilibrium(tests[2]), ans[2], "<code>equilibrium([2, 9, 2])</code> should return <code>[1]</code>.");'
  - text: '<code>equilibrium([1, -1, 1, -1, 1, -1, 1])</code> should return <code>[0,1,2,3,4,5,6]</code>.'
    testString: 'assert.deepEqual(equilibrium(tests[3]), ans[3], "<code>equilibrium([1, -1, 1, -1, 1, -1, 1])</code> should return <code>[0,1,2,3,4,5,6]</code>.");'
  - text: '<code>equilibrium([1])</code> should return <code>[0]</code>.'
    testString: 'assert.deepEqual(equilibrium(tests[4]), ans[4], "<code>equilibrium([1])</code> should return <code>[0]</code>.");'
  - text: '<code>equilibrium([])</code> should return <code>[]</code>.'
    testString: 'assert.deepEqual(equilibrium(tests[5]), ans[5], "<code>equilibrium([])</code> should return <code>[]</code>.");'

Challenge Seed

function equilibrium (a) {
  // Good luck!
}

After Test

console.info('after the test');

Solution

function equilibrium(a) {
  let N = a.length,
    i,
    l = [],
    r = [],
    e = [];
  for (l[0] = a[0], r[N - 1] = a[N - 1], i = 1; i < N; i++)
    { l[i] = l[i - 1] + a[i], r[N - i - 1] = r[N - i] + a[N - i - 1]; }
  for (i = 0; i < N; i++)
    { if (l[i] === r[i]) e.push(i); }
  return e;
}

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