add if-else, countdown and sum of evens

Rust/rustifinity/challenges/countdown.rs

@@ -0,0 +1,15 @@
+pub fn countdown(n: u32) -> Vec<u32> {
+    // TODO: Implement the countdown using a while loop
+if n == 0 {
+        vec![0]
+    } else {
+        let mut vect : Vec<u32> = Vec::new();
+        let mut m = n;
+        while m > 0 {
+            vect.push(m);
+            m -= 1;
+        }
+        vect.push(0);
+        vect
+    }
+}

Rust/rustifinity/challenges/if-else.rs

@@ -0,0 +1,8 @@
+pub fn is_even(n: i32) -> bool {
+    // Your code here...
+    if n % 2 == 0 {
+        true
+    } else {
+        false
+    }
+}

Rust/rustifinity/challenges/sum-of-even-numbers.rs

@@ -0,0 +1,9 @@
+pub fn sum_of_evens(start: i32, end: i32) -> i32 {
+    let mut sum = 0;
+    for num in start..=end + 1 {
+        if num % 2 == 0 {
+            sum += num;
+        }
+    }
+    sum
+}