add problem 42 and test function

Author: supercoderhawk

C/1-50/42-Trapping-Rain-Water.c

@@ -0,0 +1,34 @@
+/*
+ * Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.
+
+ * For example,
+ * Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.
+ * Created by supercoderx on 2017/8/13.
+ */
+#include 
+
+int trap(int *height, int heightSize) {
+    if (heightSize < 2)
+        return 0;
+    int volume = 0, maxLeft, maxRight;
+    for (int i = 0; i < heightSize; i++) {
+        maxLeft = maxRight = height[i];
+        for (int j = 0; j < i; j++) {
+            if (height[j] > maxLeft)
+                maxLeft = height[j];
+        }
+        for (int k = i + 1; k < heightSize; k++) {
+            if (height[k] > maxRight)
+                maxRight = height[k];
+        }
+        volume += (maxLeft < maxRight ? maxLeft : maxRight) - height[i];
+    }
+    return volume;
+}
+
+void testTrap() {
+    int a[] = {0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1};
+    int b[] = {5, 2, 1, 2, 1, 5};
+    printf("%d\n", trap(b, sizeof(b) / sizeof(b[0])));
+    printf("%d", trap(a, sizeof(a) / sizeof(a[0])));
+}