Add solutions to challenge 120

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+/*
+Challenge 120
+
+TASK #1 - Swap Odd/Even bits
+Submitted by: Mohammad S Anwar
+You are given a positive integer $N less than or equal to 255.
+
+Write a script to swap the odd positioned bit with even positioned bit and
+print the decimal equivalent of the new binary representation.
+
+Example
+Input: $N = 101
+Output: 154
+
+Binary representation of the given number is 01 10 01 01.
+The new binary representation after the odd/even swap is 10 01 10 10.
+The decimal equivalent of 10011010 is 154.
+
+Input: $N = 18
+Output: 33
+
+Binary representation of the given number is 00 01 00 10.
+The new binary representation after the odd/even swap is 00 10 00 01.
+The decimal equivalent of 100001 is 33.
+*/
+
+#include <iostream>
+using namespace std;
+
+int swap_bits(int n) {
+    int out = 0;
+    int shift = 0;
+    while (n > 0) {
+        if ((n & 1) != 0) { out |= 2 << shift; }
+        if ((n & 2) != 0) { out |= 1 << shift; }
+        n >>= 2;
+        shift += 2;
+    }
+    return out;
+}
+
+int main(int argc, char* argv[]) {
+    int n = 0;
+    if (argc == 2) n = atoi(argv[1]);
+    cout << swap_bits(n) << endl;
+}