add Problem24

Src.lean

@@ -14,6 +14,7 @@ import Src.Problem20
 import Src.Problem21
 import Src.Problem22
 import Src.Problem23
+import Src.Problem24
 import Src.Problem3
 import Src.Problem31
 import Src.Problem32

Src/Problem24.lean

@@ -0,0 +1,34 @@
+/-
+# Problem 24
+(Easy 🌟) Lotto: Draw `N` different random numbers from the set `1..M`.
+-/
+
+-- You can use this function
+#check List.range
+
+def diffSelect (count range : Nat) : IO (List Nat) := do
+  -- sorry
+  if count > range then
+    dbg_trace s!"can't draw {count} different numbers from 1..{range}"
+    pure []
+  else
+    let mut univ := List.range range |>.drop 1
+    let mut result : List Nat := []
+    for _ in [0 : count] do
+      let (element, rest) ← extractOne univ
+      univ := rest
+      result := element :: result
+    pure result
+where
+  extractOne (univ : List Nat) : IO (Nat × List Nat) := do
+    let index ← IO.rand 0 univ.length
+    let element := univ.get! index
+    let rest := univ.take index ++ univ.drop (index + 1)
+    pure (element, rest)
+  -- sorry
+
+#eval diffSelect 24 22
+
+#eval diffSelect 6 49
+
+#eval diffSelect 1998 1999

md/SUMMARY.md

@@ -33,6 +33,7 @@
 * [21: Insert an element](./build/Problem21.md)
 * [22: Integers within a range](./build/Problem22.md)
 * [23: Random selection](./build/Problem23.md)
+* [24: Draw N diffrent random numbers](./build/Problem24.md)
 
 # 31-40: Arithmetic