✨ feat：添加欧拉项目第30题和第31题的解决方案

📝 docs：为第31题添加详细的动态规划算法说明文档

solutions/0031/euler_31.py

@@ -0,0 +1,27 @@
+"""
+In the United Kingdom the currency is made up of pound (£) and pence (p).
+There are eight coins in general circulation:
+
+    1p, 2p, 5p, 10p, 20p, 50p, £1 (100p), and £2 (200p).
+
+It is possible to make £2 in the following way:
+
+    1×£1 + 1×50p + 2×20p + 1×5p + 1×2p + 3×1p
+
+How many different ways can £2 be made using any number of coins?
+"""
+
+
+def main():
+    coins = [1, 2, 5, 10, 20, 50, 100, 200]
+    ways = [1] + [0] * 200
+
+    for coin in coins:
+        for i in range(coin, 201):
+            ways[i] += ways[i - coin]
+
+    print(f"\nThe number of ways to make £2 is {ways[200]:,d}")
+
+
+if __name__ == "__main__":
+    main()