[Project Euler] Fix code style for multiple problems (#3094)

* Fix code style for Project Euler problems:

- 13, 17, 21
- Default args
- Type hints
- File path

* Fix code style for multiple problems

* Made suggested changes

project_euler/problem_31/sol1.py

@@ -1,6 +1,7 @@
 """
 Coin sums
-Problem 31
+Problem 31: https://projecteuler.net/problem=31
+
 In England the currency is made up of pound, £, and pence, p, and there are
 eight coins in general circulation:
 
@@ -12,39 +13,39 @@ How many different ways can £2 be made using any number of coins?
 """
 
 
-def one_pence():
+def one_pence() -> int:
     return 1
 
 
-def two_pence(x):
+def two_pence(x: int) -> int:
     return 0 if x < 0 else two_pence(x - 2) + one_pence()
 
 
-def five_pence(x):
+def five_pence(x: int) -> int:
     return 0 if x < 0 else five_pence(x - 5) + two_pence(x)
 
 
-def ten_pence(x):
+def ten_pence(x: int) -> int:
     return 0 if x < 0 else ten_pence(x - 10) + five_pence(x)
 
 
-def twenty_pence(x):
+def twenty_pence(x: int) -> int:
     return 0 if x < 0 else twenty_pence(x - 20) + ten_pence(x)
 
 
-def fifty_pence(x):
+def fifty_pence(x: int) -> int:
     return 0 if x < 0 else fifty_pence(x - 50) + twenty_pence(x)
 
 
-def one_pound(x):
+def one_pound(x: int) -> int:
     return 0 if x < 0 else one_pound(x - 100) + fifty_pence(x)
 
 
-def two_pound(x):
+def two_pound(x: int) -> int:
     return 0 if x < 0 else two_pound(x - 200) + one_pound(x)
 
 
-def solution(n):
+def solution(n: int = 200) -> int:
     """Returns the number of different ways can n pence be made using any number of
     coins?
 
@@ -61,4 +62,4 @@ def solution(n):
 
 
 if __name__ == "__main__":
-    print(solution(int(str(input()).strip())))
+    print(solution(int(input().strip())))

project_euler/problem_31/sol2.py

@@ -1,4 +1,7 @@
-"""Coin sums
+"""
+Problem 31: https://projecteuler.net/problem=31
+
+Coin sums
 
 In England the currency is made up of pound, £, and pence, p, and there are
 eight coins in general circulation:
@@ -29,7 +32,7 @@ Example:
 """
 
 
-def solution(pence: int) -> int:
+def solution(pence: int = 200) -> int:
     """Returns the number of different ways to make X pence using any number of coins.
     The solution is based on dynamic programming paradigm in a bottom-up fashion.