add new solutions

solutions/0/q42/solution.go

@@ -0,0 +1,36 @@
+package q42
+
+func trap(height []int) int {
+	water := 0
+	mid := 0
+
+	// From left
+	maxHeight, stash := 0, 0
+	for i, h := range height {
+		if h >= maxHeight {
+			maxHeight = h
+			water += stash
+			stash = 0
+			mid = i
+		} else {
+			stash += maxHeight - h
+		}
+	}
+
+	// From right
+	maxHeight, stash = 0, 0
+	for i := len(height) - 1; i >= mid; i-- {
+		h := height[i]
+		if h >= maxHeight {
+			maxHeight = h
+			water += stash
+			stash = 0
+		} else {
+			stash += maxHeight - h
+		}
+	}
+
+	return water
+}
+
+var _ = trap