diff --git a/11-42.c b/11-42.c
new file mode 100644
index 0000000..c9f3714
--- /dev/null
+++ b/11-42.c
@@ -0,0 +1,42 @@
+#include <stdio.h>
+
+int trap(int* height, int heightSize) {
+    int ans = 0;
+    int max = 0;
+    int max_x = 0;
+    for (int i = 0; i < heightSize; ++i) {
+        if (height[i] > max) {
+            max = height[i];
+            max_x = i;
+        }
+    }
+
+    // left part
+    int curleft = 0;
+    for (int i = 0; i < max_x; ++i) {
+        if (height[i] >= curleft) {
+            curleft = height[i];
+        }
+        else {
+            ans += curleft - height[i];
+        }
+    }
+
+    // right part
+    int curright = 0;
+    for (int i = heightSize - 1; i > max_x; --i) {
+        if (height[i] >= curright) {
+            curright = height[i];
+        }
+        else {
+            ans += curright - height[i];
+        }
+    }
+    return ans;
+}
+
+int main() {
+    int height[] = {4,2,0,3,2,5};
+    printf("%d\n", trap(height, sizeof(height) / sizeof(int)));
+    return 0;
+}
\ No newline at end of file