Add System Design Interview book.

problems/cons_car_cdr.py

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+"""
+cons(a, b) constructs a pair, and car(pair) and cdr(pair) return the first
+and last element of that pair.
+
+    car(cons(3, 4)) == 3
+    cdr(cons(3, 4)) == 4
+
+Given this implementation of cons, implement car and cdr.
+"""
+
+
+def cons(a, b):
+    return lambda f: f(a, b)
+
+
+def car(pair):
+    return pair(lambda a, b: a)
+
+
+def cdr(pair):
+    return pair(lambda a, b: b)
+
+
+# --- tests ---
+
+assert car(cons(3, 4)) == 3
+assert cdr(cons(3, 4)) == 4
+assert car(cons("a", "b")) == "a"
+assert cdr(cons("a", "b")) == "b"
+
+print("All tests passed!")