Find in Mountain Array LeetCode solution in JavaScript

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/**
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 * // This is the MountainArray's API interface.
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 * // You should not implement it, or speculate about its implementation
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 * function MountainArray() {
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 *     @param {number} index
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 *     @return {number}
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 *     this.get = function(index) {
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 *         ...
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 *     };
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 *
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 *     @return {number}
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 *     this.length = function() {
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 *         ...
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 *     };
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 * };
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 */
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var binarySearch = function (left, right, condition) {
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  while (left < right) {
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    var mid = left + Math.floor((right - left) / 2);
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    if (condition(mid)) {
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      right = mid;
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    } else {
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      left = mid + 1;
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    }
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  }
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  return left;
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};
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/**
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 * @param {number} target
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 * @param {MountainArray} mountainArr
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 * @return {number}
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 */
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var findInMountainArray = function (target, mountainArr) {
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  var n = mountainArr.length();
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  var maxNumIdx = binarySearch(1, n - 2, function (idx) {
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    return mountainArr.get(idx) > mountainArr.get(idx + 1);
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  });
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  var leftIdx = binarySearch(0, maxNumIdx, function (idx) {
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    return mountainArr.get(idx) >= target;
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  });
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  if (mountainArr.get(leftIdx) === target) {
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    return leftIdx;
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  }
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  var rightIdx = binarySearch(maxNumIdx, n - 1, function (idx) {
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    return mountainArr.get(idx) <= target;
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  });
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  if (mountainArr.get(rightIdx) === target) {
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    return rightIdx;
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  }
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  return -1;
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};