添加（0724.寻找数组的中心索引.md）：增加typescript版本

problems/0724.寻找数组的中心索引.md

@@ -140,6 +140,24 @@ var pivotIndex = function(nums) {
 };
 ```
 
+### TypeScript
+
+```typescript
+function pivotIndex(nums: number[]): number {
+    const length: number = nums.length;
+    const sum: number = nums.reduce((a, b) => a + b);
+    let leftSum: number = 0;
+    for (let i = 0; i < length; i++) {
+        const rightSum: number = sum - leftSum - nums[i];
+        if (leftSum === rightSum) return i;
+        leftSum += nums[i];
+    }
+    return -1;
+};
+```
+
+
+
 
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