4 files changed, 125 insertions, 2 deletions
diff --git a/challenge-070/paulo-custodio/perl/ch-1.pl b/challenge-070/paulo-custodio/perl/ch-1.pl
index fc3c102124..6c931b3227 100644
--- a/challenge-070/paulo-custodio/perl/ch-1.pl
+++ b/challenge-070/paulo-custodio/perl/ch-1.pl
@@ -2,7 +2,7 @@
 
 # Challenge 070
 #
-# TASK #1 › Character Swapping
+# TASK #1 > Character Swapping
 # Submitted by: Mohammad S Anwar
 # You are given a string $S of size $N.
 #
diff --git a/challenge-070/paulo-custodio/perl/ch-2.pl b/challenge-070/paulo-custodio/perl/ch-2.pl
index c2873d5bbb..b2106b8252 100644
--- a/challenge-070/paulo-custodio/perl/ch-2.pl
+++ b/challenge-070/paulo-custodio/perl/ch-2.pl
@@ -2,7 +2,7 @@
 
 # Challenge 070
 #
-# TASK #2 › Gray Code Sequence
+# TASK #2 > Gray Code Sequence
 # Submitted by: Mohammad S Anwar
 # You are given an integer 2 <= $N <= 5.
 #
diff --git a/challenge-070/paulo-custodio/python/ch-1.py b/challenge-070/paulo-custodio/python/ch-1.py
new file mode 100644
index 0000000000..2c3ee74ebb
--- /dev/null
+++ b/challenge-070/paulo-custodio/python/ch-1.py
@@ -0,0 +1,55 @@
+#!/usr/bin/env python3
+
+# Challenge 070
+#
+# TASK #1 > Character Swapping
+# Submitted by: Mohammad S Anwar
+# You are given a string $S of size $N.
+#
+# You are also given swap count $C and offset $O such that
+# $C >= 1, $O >= 1, $C <= $O and $C + $O <= $N.
+#
+#
+# UPDATE: 2020-07-20 16:10:00
+# Pete Houston suggested to put additional constraint i.e. $C <= $O.
+# He presented the use case $S = 'abcd', $C = 2, $O = 1.
+#
+# Write a script to perform character swapping like below:
+#
+# $S[ 1 % $N ] <=> $S[ (1 + $O) % $N ]
+# $S[ 2 % $N ] <=> $S[ (2 + $O) % $N ]
+# $S[ 3 % $N ] <=> $S[ (3 + $O) % $N ]
+# ...
+# ...
+# $S[ $C % $N ] <=> $S[ ($C + $O) % $N ]
+# Example 1
+# Input:
+#     $S = 'perlandraku'
+#     $C = 3
+#     $O = 4
+#
+# Character Swapping:
+#     swap 1: e <=> n = pnrlaedraku
+#     swap 2: r <=> d = pndlaerraku
+#     swap 3: l <=> r = pndraerlaku
+#
+# Output:
+#     pndraerlaku
+
+import sys
+
+def swap(s, c, o):
+    def swap1(s, c, o):
+        s_list = list(s)
+        p1 = c % len(s_list)
+        p2 = (c + o) % len(s_list)
+        s_list[p1], s_list[p2] = s_list[p2], s_list[p1]
+        return ''.join(s_list)
+
+    for _ in range(1, c+1):
+        s = swap1(s, _, o)
+    return s
+
+
+S, C, O = sys.argv[1], int(sys.argv[2]), int(sys.argv[3])
+print(swap(S, C, O))
diff --git a/challenge-070/paulo-custodio/python/ch-2.py b/challenge-070/paulo-custodio/python/ch-2.py
new file mode 100644
index 0000000000..96724bc983
--- /dev/null
+++ b/challenge-070/paulo-custodio/python/ch-2.py
@@ -0,0 +1,68 @@
+#!/usr/bin/env python3
+
+# Challenge 070
+#
+# TASK #2 > Gray Code Sequence
+# Submitted by: Mohammad S Anwar
+# You are given an integer 2 <= $N <= 5.
+#
+# Write a script to generate $N-bit gray code sequence.
+#
+# 2-bit Gray Code Sequence
+# [0, 1, 3, 2]
+# To generate the 3-bit Gray code sequence from the 2-bit Gray code sequence,
+# follow the step below:
+#
+# 2-bit Gray Code sequence
+# [0, 1, 3, 2]
+#
+# Binary form of the sequence
+# a) S1 = [00, 01, 11, 10]
+#
+# Reverse of S1
+# b) S2 = [10, 11, 01, 00]
+#
+# Prefix all entries of S1 with '0'
+# c) S1 = [000, 001, 011, 010]
+#
+# Prefix all entries of S2 with '1'
+# d) S2 = [110, 111, 101, 100]
+#
+# Concatenate S1 and S2 gives 3-bit Gray Code sequence
+# e) [000, 001, 011, 010, 110, 111, 101, 100]
+#
+# 3-bit Gray Code sequence
+# [0, 1, 3, 2, 6, 7, 5, 4]
+# Example
+# Input: $N = 4
+#
+# Output: [0, 1, 3, 2, 6, 7, 5, 4, 12, 13, 15, 14, 10, 11, 9, 8]
+
+import sys
+
+def gray(n):
+    if n < 2:
+        raise ValueError("N must be at least 2")
+    elif n == 2:
+        return [0, 1, 3, 2]
+    else:
+        # gray sequence of N-1
+        prev = gray(n-1)
+        # binary form to S1
+        s1 = [format(x, f'0{n-1}b') for x in prev]
+        # reverse to S2
+        s2 = s1[::-1]
+        # prefix S1 with 0
+        s1 = ['0' + x for x in s1]
+        # prefix S2 with 1
+        s2 = ['1' + x for x in s2]
+        # concatenate
+        gray_seq = s1 + s2
+        # convert to decimal
+        gray_seq = [int(x, 2) for x in gray_seq]
+
+        return gray_seq
+
+if __name__ == "__main__":
+    N = int(sys.argv[1]) if len(sys.argv) > 1 else 2
+    print(", ".join(map(str, gray(N))))