add new solutions

solutions/7/q756/solution.go

@@ -0,0 +1,55 @@
+package q756
+
+type Bottom [2]byte
+
+func canBuild(buffer []byte, layer, offset int, allowed map[Bottom][]byte) bool {
+	layerOffset := len(buffer) - (layer+1)*layer/2
+	current := buffer[layerOffset : layerOffset+layer]
+	bottom := buffer[layerOffset-layer-1 : layerOffset]
+
+	nextLayer, nextOffset := layer, offset+1
+	if nextOffset == layer {
+		nextLayer, nextOffset = layer-1, 0
+	}
+	if current[offset] != 0 { // skip
+		return canBuild(buffer, nextLayer, nextOffset, allowed)
+	}
+
+	choices := allowed[Bottom{bottom[offset], bottom[offset+1]}]
+	if len(choices) == 0 {
+		return false
+	}
+	if layer == 1 {
+		return true
+	}
+
+	next := buffer[layerOffset+layer:]
+	for _, c := range choices {
+		current[offset] = c
+		if offset > 0 {
+			next[offset-1] = 0 // needs recalculate
+		}
+		if offset < layer-1 {
+			next[offset] = 0
+		}
+
+		if canBuild(buffer, nextLayer, nextOffset, allowed) {
+			return true
+		}
+	}
+	return false
+}
+
+func pyramidTransition(bottom string, allowed []string) bool {
+	allowedIdx := map[Bottom][]byte{}
+	for i := range allowed {
+		k := [2]byte{allowed[i][0], allowed[i][1]}
+		allowedIdx[k] = append(allowedIdx[k], allowed[i][2])
+	}
+	buffer := make([]byte, (len(bottom)+1)*len(bottom)/2)
+	copy(buffer, []byte(bottom))
+
+	return canBuild(buffer, len(bottom)-1, 0, allowedIdx)
+}
+
+var _ = pyramidTransition