Solve LeetCode #518 'Coin Change II' [Medium] using backtracking.

LeetCode/src/LeetCode.Challenges/CoinChangeII/BacktrackingSolution.cs

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+namespace LeetCode.Challenges.CoinChangeII;
+
+public static class BacktrackingSolution
+{
+    public static int CoinChange(int[] coins, int amount)
+    {
+        // Counter to store the total number of combinations
+        var count = 0;
+        Backtrack(0, amount);
+        return count;
+
+        void Backtrack(int start, int remaining)
+        {
+            // Base Case: if the remaining amount is 0, we have found a valid combination.
+            if (remaining == 0)
+            {
+                count++;
+                return;
+            }
+
+            // If the remaining amount is negative, this path is invalid.
+            if (remaining < 0)
+            {
+                return;
+            }
+
+            // Iterate through the coins, starting from the current index.
+            for (var i = start; i < coins.Length; i++)
+            {
+                // Include the current coin and recurse.
+                // 'i' ensures we can reuse the current coin.
+                Backtrack(i, remaining - coins[i]); 
+            }
+        }
+    }
+}

LeetCode/src/LeetCode.Challenges/CoinChangeII/Description.md

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+# Coin Change
+
+You are given an integer array `coins` representing coins of different denominations 
+and an integer `amount` representing a total amount of money.  
+Return the number, in how many ways you can make the target amount.
+
+You may assume that you have an infinite number of each kind of coin.
+
+## Examples
+
+### Example 1:
+
+Input: `coins = [1,2]`, `amount = 5`
+Output: `3`
+Explanation:  
+`5 = 1 + 2 + 2`
+`5 = 1 + 1 + 1 + 2`
+`5 = 1 + 1 + 1 + 1 + 1`

LeetCode/tests/LeetCode.Challenges.UnitTests/CoinChangeII/BacktrackingSolutionTests.cs

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+using LeetCode.Challenges.CoinChangeII;
+using Xunit;
+using Shouldly;
+
+namespace LeetCode.Challenges.UnitTests.CoinChangeII;
+
+public class BacktrackingSolutionTests
+{
+    [Theory]
+    [MemberData(nameof(CoinChangeTestCases))]
+    public void GivenCoins_WhenCoinChange_ThenResultAsExpected(int[] coins, int amount, int expected)
+    {
+        BacktrackingSolution.CoinChange(coins, amount).ShouldBe(expected);
+    }
+
+    public static IEnumerable<object[]> CoinChangeTestCases()
+    {
+        //                      (5)
+        //                    /    \
+        //               -1 (4)     -2 (3)
+        //              /    \         \
+        //          -1 (3)   -2 (2)     -2 (1)
+        //         /    \       |          |
+        //     -1 (2)  -2 (0)   (0)        (X)
+        //    /    \
+        // -1 (1)  -2 (X)
+        //  /
+        // (0)
+        // 
+        // Valid Paths:
+        // [1, 1, 1, 1, 1], [1, 1, 1, 2], [1, 2, 2]
+        yield return [new[] { 1, 2 }, 5, 3];
+
+        //                      (5)
+        //                    /  |   \
+        //               -1 (4) -2 (3) -5 (0)
+        //              /  |        |
+        //          -1 (3) -2 (2)  -2 (1)
+        //         /  |        |      |
+        //     -1 (2)-2 (0)    (0)    (X)
+        //    /  |
+        // -1 (1)-2 (X)
+        //  /
+        // (0)
+        // 
+        // Valid Paths:
+        // [1, 1, 1, 1, 1], [1, 1, 1, 2], [1, 2, 2], [5]
+        yield return [new[] { 1, 2, 5 }, 5, 4];
+
+        yield return [new[] { 2 }, 3, 0];
+        yield return [new[] { 1, 3, 4 }, 6, 4];
+        yield return [new[] { 5 }, 5, 1];
+        yield return [new[] { 5 }, 2, 0];
+        yield return [new[] { 2, 5, 10 }, 1, 0];
+        yield return [new[] { 10, 5 }, 20, 3];
+        yield return [new[] { 1, 5, 10, 25 }, 30, 18];
+        yield return [new[] { 2, 3 }, 7, 1];
+        yield return [new[] { 1, 2, 5 }, 100, 541];
+        yield return [new[] { 7, 3, 2 }, 8, 2];
+        yield return [new[] { 1, 7, 10 }, 14, 4];
+        yield return [new[] { 1, 3, 4, 7 }, 15, 22];
+        yield return [new[] { 25, 50, 100 }, 30, 0];
+        yield return [new[] { 9, 6, 5, 1 }, 11, 6];
+    }
+}