update

Author: jasoncao11

P104. 被围绕的区域.md

@@ -0,0 +1,39 @@
+![algo43](./images/algo43.jpg)
+
+```
+class Solution:
+    def solve(self, board: List[List[str]]) -> None:
+        """
+        Do not return anything, modify board in-place instead.
+        """
+        m = len(board)
+        n = len(board[0])
+
+        def dfs(i,j):
+            if not 0<=i<m or not 0<=j<n or board[i][j] != 'O':
+                return
+            board[i][j] = 'A'
+            #情况1
+            dfs(i+1,j)
+            #情况2
+            dfs(i-1,j)
+            #情况3
+            dfs(i,j+1)
+            #情况4
+            dfs(i,j-1)             
+
+        for i in range(m):
+            dfs(i,0)
+            dfs(i,n-1)
+
+        for j in range(n):
+            dfs(0, j)
+            dfs(m-1, j)
+
+        for i in range(m):
+            for j in range(n):
+                if board[i][j] == 'A':
+                    board[i][j] = 'O'
+                elif board[i][j] == 'O':
+                    board[i][j] = 'X'
+```