js 统计逆序对的个数

// 过程：先把数组分割成子数组，先统计出子数组内部的逆序对的数目，然后再统计出两个相邻子数组之间的逆序对的数目。在统计逆序对的过程中，还需要对数组进行排序。如果对排序算法很熟悉，我们不难发现这个过程实际上就是归并排序
function InversePairs(data = []) {
  // write code here
  let len = data.length
  if (len === 0) return 0
  const copy = data.concat([])
  let count = InversePairsHelp(data, copy, 0, len - 1)
  return count
  function InversePairsHelp(data, copy, start, end) {
    if (start === end) {
      copy[start] = data[start]
      return 0
    }
    let mid = Math.floor((end - start) / 2)
    let left = InversePairsHelp(copy, data, start, start + mid)
    let right = InversePairsHelp(copy, data, start + mid + 1, end)
    let i = start + mid
    let j = end
    let count = 0
    let indexCopy = end
    while (i >= start && j >= start + mid + 1) {
      if (data[i] > data[j]) {
        copy[indexCopy--] = data[i--]
        count = count + j - start - mid
      } else {
        copy[indexCopy--] = data[j--]
      }
    }
    for (; i >= start; i--)
      copy[indexCopy--] = data[i]
    for (; j >= start + mid + 1; j--)
      copy[indexCopy--] = data[j]
    return left + right + count
  }
}