剑指 Offer 51. 数组中的逆序对

题目描述：
[51]
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。

考察重点：
第51题归并排序实现。在排序过程中，比较左右两数组元素大小时，计算逆序对个数。

第51题

class Solution {int count = 0;public int reversePairs(int[] nums) {mergeSort(nums, 0, nums.length - 1);return count;}public void mergeSort(int[] nums, int left, int right){if(left >= right)   return;int mid = left + (right - left) / 2;mergeSort(nums, left, mid);mergeSort(nums, mid + 1, right);merge(nums, left, mid, right);}public void merge(int[] nums, int left, int mid, int right){int[] numsTemp = new int[right - left + 1];int pointTemp = 0, pointR = mid + 1, pointL = left;while(pointL <= mid || pointR <= right){if(pointR > right || pointL > mid){while(pointL <= mid){numsTemp[pointTemp++] = nums[pointL ++];}while(pointR <= right){numsTemp[pointTemp++] = nums[pointR ++];}break;}if(nums[pointL] > nums[pointR]){count += (mid - pointL + 1);numsTemp[pointTemp ++] = nums[pointR++];}else{numsTemp[pointTemp ++] = nums[pointL++];}}for(int i = left;i <= right;i ++){nums[i] = numsTemp[i - left];}}
}