在数组中的两个数字，如果前面一个数字大于后面的数字，则这两个数字组成一个逆序对。输入一个数组，求出这个数组中的逆序对的总数。

示例 1:
输入: [7,5,6,4]
输出: 5

/**
 * @param {number[]} nums
 * @return {number}
 */
const reversePairs = function (nums) {
  let count = 0;
  const merge = (left, right) => {
    let res = [];
    while (left.length && right.length) {
      if (left[0] > right[0]) {
        res.push(left.shift());
        count += right.length;
      } else {
        res.push(right.shift());
      }
    }
    return res.concat(left).concat(right);
  };
  const split = (nums) => {
    if (nums.length <= 1) return nums;
    let mid = (nums.length / 2) >> 0;
    let left = nums.slice(0, mid),
      right = nums.slice(mid);
    return merge(split(left), split(right));
  };
  split(nums);
  return count;
};