[python]LCR 170. 交易逆序对的总数

README.md

@@ -1 +1,5 @@
 # code-every-day
+
+## 算法
+
+- [逆序对/归并排序](./notes/逆序对.md)

notes/逆序对.md

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+# 逆序对
+
+- [题解 1](https://leetcode.cn/problems/shu-zu-zhong-de-ni-xu-dui-lcof/solutions/622496/jian-zhi-offer-51-shu-zu-zhong-de-ni-xu-pvn2h)
+- [题解 2](https://www.bilibili.com/video/BV1Qk4y1r7u5/?spm_id_from=333.337.search-card.all.click&vd_source=fa26c1805b6d94a7f2b5002f0ae9a401)

src/leetcode/LCR00170.shu-zu-zhong-de-ni-xu-dui-lcof/170.py

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+from typing import List
+
+
+class Solution:
+    def reversePairs(self, record: List[int]) -> int:
+        """
+        https://leetcode.cn/problems/shu-zu-zhong-de-ni-xu-dui-lcof/description/
+        """
+        tmp = [0] * len(record)
+
+        def merge_sort(l, r):
+            if l >= r:
+                return 0
+            m = (l + r) >> 1
+            res = merge_sort(l, m) + merge_sort(m + 1, r)
+
+            i, j = l, m + 1
+            tmp[l : r + 1] = record[l : r + 1]
+            for k in range(l, r + 1):
+                if i == m + 1:
+                    record[k] = tmp[j]
+                    j += 1
+                elif j == r + 1 or tmp[i] <= tmp[j]:
+                    record[k] = tmp[i]
+                    i += 1
+                else:
+                    record[k] = tmp[j]
+                    j += 1
+                    res += m - i + 1
+            return res
+
+        return merge_sort(0, len(record) - 1)
+
+
+if __name__ == "__main__":
+    res = Solution().reversePairs([9, 7, 5, 4, 6])
+    print(res)