Create 1512-number-of-good-pairs.java

Author: sujal-goswami

java/1512-number-of-good-pairs.java

@@ -0,0 +1,56 @@
+/*    Brute Force (Method 1)
+-----------------------------
+  Time Complexity: O(n^2)
+  Space Complexity: O(1)
+----------------------------*/
+class Solution {
+   public int numIdenticalPairs(int[] nums) {
+        int i=0,j=0,c=0;     
+        for(i=0;i<nums.length;i++){
+            for(j=i+1;j<nums.length;j++){
+                if(nums[i]==nums[j])
+                    c++;;
+            }
+        }
+        return c;
+    }
+}
+
+/*   Combinations (Method 2)
+---------------------------------
+  Time Complexity: O(n)
+  Space Complexity: O(n)
+---------------------------------*/
+class Solution {
+    public int numIdenticalPairs(int[] nums) {
+        Map<Integer, Integer> freq = new HashMap<>();
+        for(int n: nums)
+            freq.put(n, freq.getOrDefault(n, 0) + 1);
+        int res = 0;
+        for(int c : freq.values()){
+            res += c*(c-1)/2;    // Combination formula nC2 = n*n(n-1)/2
+        }
+        return res;    
+    }
+}
+
+/*         Method 3
+----------------------------
+  Time Complexity: O(n)
+  Space Complexity: O(n)
+---------------------------*/
+class Solution {
+    public int numIdenticalPairs(int[] nums) {
+        int res = 0;
+        Map<Integer, Integer> freq = new HashMap<>();
+        for(int n : nums){
+            if(freq.containsKey(n)){
+                res += freq.get(n);
+                freq.put(n, freq.get(n)+1);
+            }
+            else
+                freq.put(n, 1);
+        }
+        return res;   
+    }
+}