923

Author: ProHiryu

Math/923.md

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+## 3Sum With Multiplicity
+
+#### Description
+
+[link](https://leetcode.com/problems/3sum-with-multiplicity/)
+
+---
+
+#### Solution
+
+- See Code
+
+---
+
+#### Code
+
+> Complexity T : O(n) M : O(n)
+
+```python
+class Solution:
+    def threeSumMulti(self, arr: List[int], target: int) -> int:
+        cnt, ans = collections.Counter(arr), 0
+        for i in cnt:
+            for j in cnt:
+                k = target - i - j
+                if i == j == k:
+                    ans += cnt[i] * (cnt[i] - 1) * (cnt[i] - 2) // 6
+                elif i == j:
+                    ans += cnt[i] * (cnt[i] - 1) // 2 * cnt[k]
+                elif i < j < k:
+                    ans += cnt[i] * cnt[j] * cnt[k]
+        return ans % (10**9 + 7)
+```