feat: add typescript solution to locf problem: No.51. Reverse Pairs (doocs#436)

Author: zhaocchen

lcof/面试题51. 数组中的逆序对/README.md

@@ -192,6 +192,50 @@ private:
 };
 ```
 
+### **TypeScript**
+
+```ts
+function reversePairs(nums: number[]): number {
+    let count: number = 0;
+    const n: number = nums.length;
+    if (n < 2) return 0;
+
+    function merge(nums: number[], left: number, mid: number, right: number): void {
+        let n: number = right - left + 1;
+        let t: number[] = new Array(n);
+        let i: number = left, j: number = mid + 1, idx: number = 0;
+        while (i <= mid && j <= right) {
+            if (nums[i] > nums[j]) {
+                count += (mid - i + 1);
+                t[idx++] = nums[j++];
+            } else {
+                t[idx++] = nums[i++];
+            }
+        }
+        while (i <= mid) {
+            t[idx++] = nums[i++];
+        }
+        while (j <= right) {
+            t[idx++] = nums[j++];
+        }
+        for (let k: number = 0; k < n; ++k) {
+            nums[left + k] = t[k];
+        }
+    }
+    
+    function mergeSort(nums: number[], left: number, right: number): void {
+        if (left == right) return;
+        let mid: number = (left + right) >> 1;
+        mergeSort(nums, left, mid);
+        mergeSort(nums, mid + 1, right);
+        merge(nums, left, mid, right);
+    }
+
+    mergeSort(nums, 0, n - 1);
+    return count;
+};
+```
+
 ### **...**
 
 ```

lcof/面试题51. 数组中的逆序对/Solution.ts

@@ -0,0 +1,39 @@
+function reversePairs(nums: number[]): number {
+    let count: number = 0;
+    const n: number = nums.length;
+    if (n < 2) return 0;
+
+    function merge(nums: number[], left: number, mid: number, right: number): void {
+        let n: number = right - left + 1;
+        let t: number[] = new Array(n);
+        let i: number = left, j: number = mid + 1, idx: number = 0;
+        while (i <= mid && j <= right) {
+            if (nums[i] > nums[j]) {
+                count += (mid - i + 1);
+                t[idx++] = nums[j++];
+            } else {
+                t[idx++] = nums[i++];
+            }
+        }
+        while (i <= mid) {
+            t[idx++] = nums[i++];
+        }
+        while (j <= right) {
+            t[idx++] = nums[j++];
+        }
+        for (let k: number = 0; k < n; ++k) {
+            nums[left + k] = t[k];
+        }
+    }
+    
+    function mergeSort(nums: number[], left: number, right: number): void {
+        if (left == right) return;
+        let mid: number = (left + right) >> 1;
+        mergeSort(nums, left, mid);
+        mergeSort(nums, mid + 1, right);
+        merge(nums, left, mid, right);
+    }
+
+    mergeSort(nums, 0, n - 1);
+    return count;
+};