添加（0454.四数相加II.md）:增加typescript版本

Author: xiaofei-2020

problems/0454.四数相加II.md

@@ -139,7 +139,7 @@ class Solution(object):
         return count
 
 
-``` 
+```
 
 Go：
 ```go
@@ -192,8 +192,33 @@ var fourSumCount = function(nums1, nums2, nums3, nums4) {
 };
 ```
 
+TypeScript：
+
+```typescript
+function fourSumCount(nums1: number[], nums2: number[], nums3: number[], nums4: number[]): number {
+    let helperMap: Map<number, number> = new Map();
+    let resNum: number = 0;
+    let tempVal: number | undefined;
+    for (let i of nums1) {
+        for (let j of nums2) {
+            tempVal = helperMap.get(i + j);
+            helperMap.set(i + j, tempVal ? tempVal + 1 : 1);
+        }
+    }
+    for (let k of nums3) {
+        for (let l of nums4) {
+            tempVal = helperMap.get(0 - (k + l));
+            if (tempVal) {
+                resNum += tempVal;
+            }
+        }
+    }
+    return resNum;
+};
+```
 
 PHP:
+
 ```php
 class Solution {
     /**