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title	id	challengeType	isHidden	forumTopicId
Equilibrium index	5987fd532b954e0f21b5d3f6	5	false	302255

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 $A$.

Instructions

Write a function that, given a sequence, returns its equilibrium indices (if any). Assume that the sequence may be very long.

Tests

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

Challenge Seed

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

After Test

const equilibriumTests =
  [[-7, 1, 5, 2, -4, 3, 0], // 3, 6
  [2, 4, 6], // empty
  [2, 9, 2], // 1
  [1, -1, 1, -1, 1, -1, 1], // 0,1,2,3,4,5,6
  [1], // 0
  [] // empty
  ];
const ans = [[3, 6], [], [1], [0, 1, 2, 3, 4, 5, 6], [0], []];

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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