Finding length of the sequence

My Solution

var lengthOfSequence = function (arr, n) {
  var last = arr.lastIndexOf(n),
      first = arr.indexOf(n);

  if ( last === -1 || first === -1 || arr.length === 1 ||  arr.slice(first+1).indexOf(n) + first + 1 !== last) {
    return 0;
  }

  return last - first + 1;
};

하지만…

filter를 이용하면 좀 더 쉬울 수 있겠다.

var lengthOfSequence = function (arr, n) {
  return arr.filter(function(v){ return v == n }).length == 2 ? arr.lastIndexOf(n) - arr.indexOf(n) + 1 : 0
};
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