cpp: add «42. Trapping Rain Water»

Signed-off-by: Matej Focko <mfocko@redhat.com>

cpp/trapping-rain-water.cpp

@@ -0,0 +1,26 @@
+#include <algorithm>
+#include <vector>
+
+class Solution {
+  public:
+    int trap(const std::vector<int> &height) {
+        int caught = 0;
+
+        auto i = 0u;
+        auto j = height.size() - 1;
+
+        int max_l = height[i], max_r = height[j];
+        while (i < j) {
+            max_l = std::max(max_l, height[i]);
+            max_r = std::max(max_r, height[j]);
+
+            if (max_l < max_r) {
+                caught += max_l - height[i++];
+            } else {
+                caught += max_r - height[j--];
+            }
+        }
+
+        return caught;
+    }
+};