逆向思维的dfs，比如填充，实际上找到不用填充的即可，

中间往两边排水，可以转化为两边往中间灌水。
dfs由外而内更好做。可以逐渐缩小范围。

Author: h2pl

src/DFS/太平洋和大西洋灌水.java

@@ -0,0 +1,68 @@
+package DFS;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Created by 周杰伦 on 2018/3/31.
+ * 逆向思维。
+ * 先把靠着大西洋的两条边灌满大西洋水。
+ * 再把靠着太平洋的两条边灌满太平洋水。
+ * 然后往依次灌水，wet和visit表示已灌水和已遍历。
+ * 最后判断两种水都有的点极为所求。
+ * 妙啊，妙啊。
+ */
+public class 太平洋和大西洋灌水 {
+    public List<int[]> pacificAtlantic(int[][] matrix) {
+        List<int[]> result = new ArrayList<int[]>();
+        if(matrix.length == 0 || matrix[0].length == 0) return result;
+        boolean[][] pacific = new boolean[matrix.length][matrix[0].length];  // the pacific boolean table
+        boolean[][] atlantic = new boolean[matrix.length][matrix[0].length]; // the atlantic booean table
+        //initially, all the top and left cells are flooded with pacific water
+        //and all the right and bottom cells are flooded with atlantic water
+        for(int i = 0; i < matrix.length; i++){
+            pacific[i][0] = true;
+            atlantic[i][matrix[0].length-1] = true;
+        }
+        for(int i = 0; i < matrix[0].length; i++){
+            pacific[0][i] = true;
+            atlantic[matrix.length-1][i] = true;
+        }
+        //we go around the matrix and try to flood the matrix from 4 side.
+        for(int i = 0; i < matrix.length; i++){
+            boolean[][] pacificVisited = new boolean[matrix.length][matrix[0].length];
+            boolean[][] atlanticVisited = new boolean[matrix.length][matrix[0].length];
+            water(pacific, pacificVisited, matrix, i,0);
+            water(atlantic, atlanticVisited, matrix, i, matrix[0].length - 1);
+        }
+        for(int i = 0; i < matrix[0].length; i++){
+            boolean[][] pacificVisited = new boolean[matrix.length][matrix[0].length];
+            boolean[][] atlanticVisited = new boolean[matrix.length][matrix[0].length];
+            water(pacific, pacificVisited, matrix, 0,i);
+            water(atlantic, atlanticVisited, matrix, matrix.length - 1, i);
+        }
+        //check the shared points among 2 tables
+        for(int i = 0; i < matrix.length; i++){
+            for(int j = 0; j < matrix[0].length; j++){
+                if(pacific[i][j] && atlantic[i][j]){
+                    int[] element = {i,j};
+                    result.add(element);
+                }
+            }
+        }
+        return result;
+    }
+    //the flood function
+    private void water(boolean[][] wet, boolean[][] visited, int[][] matrix, int i , int j){
+        wet[i][j] = true;
+        visited[i][j] = true;
+        int[] x = {0,0,1,-1};
+        int[] y = {1,-1,0,0};
+        for(int k = 0; k < 4; k++){
+            if(i+y[k] >= 0 && i+y[k] < matrix.length && j+x[k] >= 0 && j+x[k] < matrix[0].length
+                    && !visited[i+y[k]][j+x[k]] && matrix[i+y[k]][j+x[k]] >= matrix[i][j]){
+                water(wet, visited, matrix, i+y[k], j+x[k]);
+            }
+        }
+    }
+}