Implement number-of-good-pairs

name: number-of-good-pairs
url: https://leetcode.com/problems/number-of-good-pairs
difficulty: easy

time: 0ms
time-rank: 100%
time-complexity: O(N)

space: 7.5MB
space-rank: 94.13%
space-complexity: O(1)

Fixes #279

Signed-off-by: Christopher Friedt <[email protected]>

Author: cfriedt

number-of-good-pairs-test.cpp

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2021, Christopher Friedt
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <gtest/gtest.h>
+
+#include "number-of-good-pairs.cpp"
+
+using namespace std;
+
+TEST(NumberOfGoodPairs, 1_2_3_1_1_3) {
+	vector<int> nums = {
+		1,2,3,1,1,3,
+	};
+	EXPECT_EQ(4, Solution().numIdenticalPairs(nums));
+}
+
+TEST(NumberOfGoodPairs, 1_1_1_1) {
+	vector<int> nums = {
+		1,1,1,1,
+	};
+	EXPECT_EQ(6, Solution().numIdenticalPairs(nums));
+}
+
+TEST(NumberOfGoodPairs, 1_2_3) {
+	vector<int> nums = {
+		1,2,3,
+	};
+	EXPECT_EQ(0, Solution().numIdenticalPairs(nums));
+}
+
+TEST(NumberOfGoodPairs, 6_5_1_5_7_7_9_1_5_7_1_6_10_9_7_4_1_8_7_1_1_8_6_4_7_4_10_5_3_9_10_1_9_5_5_4_1_7_4_2_9_2_6_6_4_2_10_3_5_3_6_4_7_4_6_4_4_6_3_4_10_1_10_6_10_4_9_6_6_4_8_6_9_5_4) {
+	vector<int> nums = {
+6,5,1,5,7,7,9,1,5,7,1,6,10,9,7,4,1,8,7,1,1,8,6,4,7,4,10,5,3,9,10,1,9,5,5,4,1,7,4,2,9,2,6,6,4,2,10,3,5,3,6,4,7,4,6,4,4,6,3,4,10,1,10,6,10,4,9,6,6,4,8,6,9,5,4
+	};
+	EXPECT_EQ(303, Solution().numIdenticalPairs(nums));
+}

number-of-good-pairs.cpp

@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2021, Christopher Friedt
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <sstream>
+
+#include <unordered_map>
+#include <vector>
+
+using namespace std;
+
+// name: number-of-good-pairs
+// url: https://leetcode.com/problems/number-of-good-pairs
+// difficulty: 1
+
+static int choose(int n, int k) {
+
+    // C(n, k) := n! / (k!*(n - k)!)
+    //     := n(n-1)(n-2)..(n-k+1)/(k(k-1)(k-2)..1)
+    // C(n, k) == C(n, k - n) (binomial theorem)
+    if (n - k < k) {
+        k = n - k;
+    }
+    
+    int val = 1;
+
+    for(int i = 0; i < k; ++i) {
+	val *= (n - i);
+	val /= (i + 1);
+    }
+
+    return val;
+}
+
+class Solution {
+public:
+    int numIdenticalPairs(vector<int>& nums) {
+        array<int,101> hist;
+        hist.fill(0);
+	    array<int,101> dp;
+        dp.fill(0);
+
+        // O(N)
+        for(auto& n: nums) {
+	    hist[n]++;
+        }
+
+	int r = 0;
+        for(const auto& n: hist) {
+            if (n >= 2) {
+                if (dp[n] == 0) {
+                    dp[n] = choose(n, 2);
+                }
+                r += dp[n];
+            }            
+        }
+        
+        return r;
+    }
+};
+