feat: add mock function for trap

Author: bujosa

src/hard/trapping_rain_water.rs

@@ -0,0 +1,16 @@
+#![allow(dead_code)]
+
+pub fn trap(height: Vec<i32>) -> i32 {
+    todo!("Implement trap function")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_trap() {
+        let height = vec![0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1];
+        assert_eq!(trap(height), 6);
+    }
+}