Pacific Atlantic Water Flow

Author: TonyKim9401

pacific-atlantic-water-flow/TonyKim9401.java

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+// TC: O(n * m)
+// visit all elements
+// SC: O(n * m)
+// create result from all elements
+class Solution {
+    public List<List<Integer>> pacificAtlantic(int[][] heights) {
+        List<List<Integer>> output = new ArrayList<>();
+
+        if (heights.length == 0 || heights[0].length == 0) return output;
+
+        int rows = heights.length;
+        int cols = heights[0].length;
+
+        boolean[][] pac = new boolean[rows][cols];
+        boolean[][] atl = new boolean[rows][cols];
+
+        for (int j = 0; j < rows; j++) {
+            dfs(j, 0, pac, heights[j][0], heights);
+            dfs(j, cols-1, atl, heights[j][cols-1], heights);
+        }
+
+        for (int i = 0; i < cols; i++) {
+            dfs(0, i, pac, heights[0][i], heights);
+            dfs(rows-1, i, atl, heights[rows-1][i], heights);
+        }
+
+        for (int i = 0; i < rows; i++) {
+            for (int j = 0; j < cols; j++) {
+                if (pac[i][j] && atl[i][j]) output.add(List.of(i,j));
+            }
+        }
+        return output;
+    }
+
+    private void dfs(int i, int j, boolean[][] visit, int preValue, int[][] heights) {
+        if (i < 0 || j < 0 || i == heights.length || j == heights[0].length || visit[i][j] || preValue > heights[i][j]) return;
+
+        visit[i][j] = true;
+        dfs(i + 1, j, visit, heights[i][j], heights);
+        dfs(i - 1, j, visit, heights[i][j], heights);
+        dfs(i, j + 1, visit, heights[i][j], heights);
+        dfs(i, j - 1, visit, heights[i][j], heights);
+    }
+}