Add more solutions

Added solutions for problem 206 in C and python, and for problem
357 in C.

Python/p206.py

@@ -0,0 +1,46 @@
+#!/usr/bin/python3
+
+# Find the unique positive integer whose square has the form 1_2_3_4_5_6_7_8_9_0,
+# where each “_” is a single digit.
+
+from timeit import default_timer
+
+def main():
+    start = default_timer()
+
+#   Since the square of n has 19 digits, n must be at least 10^9.
+    n = 1000000000
+    found = 0
+
+    while not found:
+#       If the square on n ends with 10, n must be divisible by 10.
+        n = n + 10
+        p = n * n
+
+#       A square divisible by 10 is also divisible by 100.
+        if p % 100 != 0:
+            continue
+
+#       Check if the digits of the square correspond to the given pattern.
+        i = 9
+        p = p // 100
+
+        while p > 0:
+            if p % 10 != i:
+                break
+            p = p // 100
+            i = i - 1
+
+        if p == 0:
+            found = 1
+
+    end = default_timer()
+
+    print('Project Euler, Problem 206')
+    print('Answer: {}'.format(n))
+
+    print('Elapsed time: {:.9f} seconds'.format(end - start))
+
+if __name__ == '__main__':
+    main()
+