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/**
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 * @param {number[]} nums
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 * @return {number}
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 */
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var reversePairs = function (nums) {
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  let numReversePairs = 0;
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  helper(nums);
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  return numReversePairs;
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  function helper(nums) {
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    if (nums.length <= 1) return nums;
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    const length = nums.length;
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    const left = helper(nums.slice(0, Math.floor(length / 2)));
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    const right = helper(nums.slice(Math.floor(length / 2)));
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    return merge(left, right);
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  }
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  function merge(left, right) {
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    const nums_sorted = [];
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    let leftIndex = 0;
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    let rightIndex = 0;
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    while (leftIndex < left.length && rightIndex < right.length) {
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      if (left[leftIndex] > 2 * right[rightIndex]) {
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        numReversePairs += left.length - leftIndex;
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        rightIndex++;
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      } else {
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        leftIndex++;
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      }
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    }
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    leftIndex = 0;
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    rightIndex = 0;
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    while (leftIndex < left.length && rightIndex < right.length) {
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      if (left[leftIndex] < right[rightIndex]) {
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        nums_sorted.push(left[leftIndex]);
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        leftIndex++;
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      } else {
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        let cur = leftIndex;
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        nums_sorted.push(right[rightIndex]);
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        rightIndex++;
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      }
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    }
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    while (leftIndex < left.length) {
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      nums_sorted.push(left[leftIndex]);
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      leftIndex++;
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    }
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    while (rightIndex < right.length) {
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      nums_sorted.push(right[rightIndex]);
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      rightIndex++;
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    }
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    return nums_sorted;
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  }
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};