5.17

Author: JiayangWu

0417.pacific-atlantic-water-flow/pacific-atlantic-water-flow.md

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+<p>Given an <code>m x n</code> matrix of non-negative integers representing the height of each unit cell in a continent, the "Pacific ocean" touches the left and top edges of the matrix and the "Atlantic ocean" touches the right and bottom edges.</p>
+
+<p>Water can only flow in four directions (up, down, left, or right) from a cell to another one with height equal or lower.</p>
+
+<p>Find the list of grid coordinates where water can flow to both the Pacific and Atlantic ocean.</p>
+
+<p><b>Note:</b><br />
+<ol>
+<li>The order of returned grid coordinates does not matter.</li>
+<li>Both <i>m</i> and <i>n</i> are less than 150.</li>
+</ol>
+</p>
+<p><b>Example:</b>
+<pre>
+Given the following 5x5 matrix:
+
+  Pacific ~   ~   ~   ~   ~ 
+       ~  1   2   2   3  (5) *
+       ~  3   2   3  (4) (4) *
+       ~  2   4  (5)  3   1  *
+       ~ (6) (7)  1   4   5  *
+       ~ (5)  1   1   2   4  *
+          *   *   *   *   * Atlantic
+
+Return:
+
+[[0, 4], [1, 3], [1, 4], [2, 2], [3, 0], [3, 1], [4, 0]] (positions with parentheses in above matrix).
+</pre>
+</p>

0417.pacific-atlantic-water-flow/pacific-atlantic-water-flow.py

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+class Solution(object):
+    def pacificAtlantic(self, matrix):
+        """
+        :type matrix: List[List[int]]
+        :rtype: List[List[int]]
+        """
+ 
+        if not matrix or not matrix[0]:
+            return list()
+        m, n = len(matrix), len(matrix[0])
+        
+        po = [[0 for i  in range(n)] for j in range(m)] #po[i][j] = 1就代表matrix[i][j]可以被太平洋的水流到
+        ao = [[0 for i  in range(n)] for j in range(m)] #同上，换成大西洋
+        
+        
+        dx = [1, -1, 0, 0]
+        dy = [0, 0, 1, -1]
+        
+        def dfs(x0, y0, string):
+            if visited[x0][y0] == 1:#来过了
+                return     
+            visited[x0][y0] = 1
+            
+            if string == "po":
+                po[x0][y0] = 1
+            else:
+                ao[x0][y0] = 1
+                
+            for k in range(4):
+                x = x0 + dx[k]
+                y = y0 + dy[k]
+                
+                if 0<= x < m and 0 <= y < n and matrix[x][y] >= matrix[x0][y0]: #可以流到下一个点
+                    dfs(x, y, string)
+            
+        visited = [[0 for i in range(n)]  for j in range(m)]
+        i = 0
+        for j in range(n):
+            dfs(i, j, "po") #上面的太平洋
+            
+        visited = [[0 for i in range(n)]  for j in range(m)]    
+        i = m - 1
+        for j in range(n):
+            dfs(i, j, "ao") #下面的大西洋
+            
+        visited = [[0 for i in range(n)]  for j in range(m)]        
+        j = 0
+        for i in range(m):
+            dfs(i, j, "po") #左边的太平洋
+            
+        visited = [[0 for i in range(n)]  for j in range(m)]    
+        j = n - 1
+        for i in range(m):
+            dfs(i, j, "ao") #右边的大西洋
+            
+        res = []
+        for i in range(m):
+            for j in range(n):
+                if po[i][j] and ao[i][j]: #取交集
+                    res.append([i, j])
+                    
+        return res