add solution to problem 923

Author: He Hao

leetcode_python_solutions/923.py

@@ -0,0 +1,31 @@
+class Solution:
+    def threeSumMulti(self, arr: List[int], target: int) -> int:
+        arr.sort()
+        ans = 0
+        MOD = 10**9 + 7
+        for i in range(len(arr)):
+            new_sum = target - arr[i]
+            left, right = i+1, len(arr)-1
+            while left < right:
+                if arr[left] + arr[right] < new_sum:
+                    left += 1
+                elif arr[left] + arr[right] > new_sum:
+                    right -= 1
+                else:
+                    if arr[left] == arr[right]:
+                        ans += (right-left+1)*(right-left)//2
+                        ans %= MOD
+                        break
+                    else:
+                        n_left = n_right = 1
+                        while left < right and arr[left] == arr[left+1]:
+                            left += 1
+                            n_left += 1
+                        while left < right and arr[right] == arr[right-1]:
+                            right -= 1
+                            n_right += 1
+                        ans += n_left*n_right
+                        ans %= MOD
+                        left += 1
+                        right -=1
+        return ans