Create 1512_Number_of_Good_Pairs.md

Author: koji

1512_Number_of_Good_Pairs.md

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+Given an array of integers nums.
+
+A pair (i,j) is called good if nums[i] == nums[j] and i < j.
+
+Return the number of good pairs.
+
+Example 1:
+
+Input: nums = [1,2,3,1,1,3]
+Output: 4
+Explanation: There are 4 good pairs (0,3), (0,4), (3,4), (2,5) 0-indexed.
+Example 2:
+
+Input: nums = [1,1,1,1]
+Output: 6
+Explanation: Each pair in the array are good.
+Example 3:
+
+Input: nums = [1,2,3]
+Output: 0
+ 
+
+Constraints:
+
+1 <= nums.length <= 100
+1 <= nums[i] <= 100
+
+0. create a counter 
+1. create a hash table: a key is number 
+2. if key's value > 1 nCr r=2 and add the result to counter
+3. return counter 
+
+```python
+from math import factorial
+def calc_pattern(self, n):
+ return factorial(n) // factorial(2) // factorial(n - 2)
+ 
+def numIdenticalPairs(self, nums: List[int]) -> int:
+ from collections import Counter
+ dic = Counter(nums)
+ counter = 0
+ for k,v in dic.items():
+ if v >1:
+ counter += self.calc_pattern(v)
+ return counter
+```