Update 3.insertionSort.md

Author: LokiSharp

3.insertionSort.md

@@ -34,4 +34,18 @@ function insertionSort(arr) {
     }
     return arr;
 }
-```
+```
+
+## 3. Python 代码实现
+
+```python
+def insertionSort(arr):
+    for i in range(len(arr)):
+        preIndex = i-1
+        current = arr[i]
+        while preIndex >= 0 and arr[preIndex] > current:
+            arr[preIndex+1] = arr[preIndex]
+            preIndex-=1
+        arr[preIndex+1] = current
+    return arr
+'''