feat: add trapping rain water solution

Author: bujosa

src/hard/trapping_rain_water.rs

@@ -1,7 +1,31 @@
 #![allow(dead_code)]
 
 pub fn trap(height: Vec<i32>) -> i32 {
-    todo!("Implement trap function")
+    let mut left = 0;
+    let mut right = height.len() - 1;
+    let mut max_capacity = 0;
+    let mut left_max = 0;
+    let mut right_max = 0;
+
+    while left < right {
+        if height[left] < height[right] {
+            if height[left] >= left_max {
+                left_max = height[left];
+            } else {
+                max_capacity += left_max - height[left];
+            }
+            left += 1;
+        } else {
+            if height[right] >= right_max {
+                right_max = height[right];
+            } else {
+                max_capacity += right_max - height[right];
+            }
+            right -= 1;
+        }
+    }
+
+    return max_capacity;
 }
 
 #[cfg(test)]