feat: osijvrsoi

pkg/questions/pacific_atlantic_water_flow/main.go

@@ -0,0 +1,105 @@
+package pacificatlanticwaterflow
+
+type Position struct{ X, Y int }
+
+func (p Position) Neighbors() []Position {
+	return []Position{
+		{p.X + 1, p.Y},
+		{p.X - 1, p.Y},
+		{p.X, p.Y + 1},
+		{p.X, p.Y - 1},
+	}
+}
+
+func (p Position) ToList() []int {
+	return []int{p.Y, p.X}
+}
+
+type Board [][]int
+
+func (b Board) Get(p Position) int {
+	if b.OutOfBounds(p) {
+		return -1
+	}
+
+	return b[p.Y][p.X]
+}
+
+func (b Board) Height() int { return len(b) }
+func (b Board) Width() int  { return len(b[0]) }
+
+func (b Board) OutOfBounds(p Position) bool {
+	return p.Y < 0 || p.Y >= b.Height() || p.X < 0 || p.X >= b.Width()
+}
+
+type Flow struct{ Pacific, Atlantic bool }
+
+func (f *Flow) Merge(o Flow) {
+	f.Atlantic = f.Atlantic || o.Atlantic
+	f.Pacific = f.Pacific || o.Pacific
+}
+
+func (f Flow) Both() bool {
+	return f.Pacific && f.Atlantic
+}
+
+func (b Board) GetFlow(p Position) Flow {
+	return Flow{
+		Atlantic: p.Y >= b.Height() || p.X >= b.Width(),
+		Pacific:  p.Y < 0 || p.X < 0,
+	}
+}
+
+func pacificAtlanticHelper(board Board, position Position, flows map[Position]Flow) Flow {
+	if flow, ok := flows[position]; ok {
+		return flow
+	}
+
+	if board.OutOfBounds(position) {
+		return board.GetFlow(position)
+	}
+
+	flow := Flow{}
+
+	var top Position
+	plateau := map[Position]bool{}
+	unvisited := []Position{position}
+	for len(unvisited) > 0 {
+		top, unvisited = unvisited[len(unvisited)-1], unvisited[:len(unvisited)-1]
+
+		if board.Get(top) < board.Get(position) {
+			flow.Merge(pacificAtlanticHelper(board, top, flows))
+		} else if board.Get(top) == board.Get(position) {
+			if _, ok := plateau[top]; !ok {
+				plateau[top] = true
+				unvisited = append(unvisited, top.Neighbors()...)
+			}
+		}
+	}
+
+	for mem := range plateau {
+		flows[mem] = flow
+	}
+
+	return flow
+}
+
+func PacificAtlantic(heights [][]int) [][]int {
+	flows := map[Position]Flow{}
+	board := Board(heights)
+
+	for y := range board.Height() {
+		for x := range board.Width() {
+			pacificAtlanticHelper(board, Position{x, y}, flows)
+		}
+	}
+
+	result := [][]int{}
+	for position, flow := range flows {
+		if flow.Both() {
+			result = append(result, position.ToList())
+		}
+	}
+
+	return result
+}