add: 只出现一次的数字等

src/array/reverse-pairs.js

@@ -0,0 +1,53 @@
+let count = 0
+
+export const resetCount = () => {
+  count = 0
+}
+
+/**
+ * 归并排序 - 合并左右
+ * @param {number[]} left
+ * @param {number[]} right
+ */
+export const merge = (left, right) => {
+  const res = []
+  const leftLength = left.length
+  const rightLength = right.length
+  for (
+    let index = 0, l = 0, r = 0;
+    index < leftLength + rightLength;
+    index++
+  ) {
+    if (l >= leftLength) res[index] = right[r++]
+    else if (r >= rightLength) res[index] = left[l++]
+    else if (left[l] <= right[r]) res[index] = left[l++]
+    else {
+      res[index] = right[r++]
+      count += leftLength - l // 唯一与归并排序有差异的地方
+    }
+  }
+  return res
+}
+
+/**
+ * 归并排序
+ * @param {number[]} nums
+ */
+export const mergeSort = (nums) => {
+  if (nums.length < 2) return nums
+  const mid = ~~(nums.length / 2)
+  const left = nums.slice(0, mid)
+  const right = nums.slice(mid)
+  return merge(mergeSort(left), mergeSort(right))
+}
+
+/**
+ * 逆序对
+ * @param {number[]} nums
+ * @return {number}
+ */
+export const reversePairs = function (nums) {
+  resetCount()
+  mergeSort(nums)
+  return count
+}