update

medium/417.pacific-atlantic-water-flow.go

@@ -2,30 +2,42 @@ var dx []int = []int{1, 0, -1, 0}
 var dy []int = []int{0, -1, 0, 1}
 
 func pacificAtlantic(matrix [][]int) (res [][]int) {
-	m, n := len(matrix), len(matrix[0])
+	m := len(matrix)
+	if m == 0 {
+		return
+	}
+	n := len(matrix[0])
+	if n == 0 {
+		return
+	}
 	ocean := make([][]int, m)
 	for i := 0; i < m; i++ {
 		ocean[i] = make([]int, n)
 	}
 	for i := 0; i < m; i++ { // b10: pacific, b01: atlantic
-		ocean[i][0] |= 2
-		ocean[i][n-1] |= 1
+		dfs(matrix, 2, 0, i, ocean, m, n)
+		dfs(matrix, 1, n-1, i, ocean, m, n)
 	}
 	for i := 0; i < n; i++ {
-		ocean[0][i] |= 2
-		ocean[m-1][i] |= 1
+		dfs(matrix, 2, i, 0, ocean, m, n)
+		dfs(matrix, 1, i, m-1, ocean, m, n)
 	}
-	dfs(matrix, 0, 0, ocean)
 	for i := 0; i < m; i++ {
-		for j := 0; j < m; j++ {
+		for j := 0; j < n; j++ {
 			if ocean[i][j] == 3 {
-				res = append(res, []int{j, i})
+				res = append(res, []int{i, j})
 			}
 		}
 	}
 	return
 }
 
-func dfs(matrix [][]int, x, y int, ocean [][]int) {
-	
+func dfs(matrix [][]int, mask, x, y int, ocean [][]int, m, n int) {
+	ocean[y][x] |= mask
+	for i := 0; i < 4; i++ {
+		nx, ny := x+dx[i], y+dy[i]
+		if 0 <= nx && nx < n && 0 <= ny && ny < m && ocean[ny][nx]&mask == 0 && matrix[y][x] <= matrix[ny][nx] {
+			dfs(matrix, mask, nx, ny, ocean, m, n)
+		}
+	}
 }