update 37. 解数独: 提供python3 版本，简化isValid， 提高回溯函数的可读性

problems/0037.解数独.md

@@ -321,6 +321,59 @@ class Solution:
         backtrack(board)
 ```
 
+Python3:
+
+```python3
+class Solution:
+    def __init__(self) -> None:
+        self.board = []
+
+    def isValid(self, row: int, col: int, target: int) -> bool:
+        for idx in range(len(self.board)):
+            # 同列是否重复
+            if self.board[idx][col] == str(target):
+                return False
+            # 同行是否重复
+            if self.board[row][idx] == str(target):
+                return False
+            # 9宫格里是否重复
+            box_row, box_col = (row // 3) * 3 + idx // 3, (col // 3) * 3 + idx % 3
+            if self.board[box_row][box_col] == str(target):
+                return False
+        return True
+    
+    def getPlace(self) -> List[int]:
+        for row in range(len(self.board)):
+            for col in range(len(self.board)):
+                if self.board[row][col] == ".":
+                    return [row, col]
+        return [-1, -1]
+    
+    def isSolved(self) -> bool:
+        row, col = self.getPlace() # 找个空位置
+        
+        if row == -1 and col == -1: # 没有空位置，棋盘被填满的
+            return True
+        
+        for i in range(1, 10):
+            if self.isValid(row, col, i): # 检查这个空位置放i，是否合适
+                self.board[row][col] = str(i) # 放i
+                if self.isSolved(): # 合适，立刻返回, 填下一个空位置。
+                    return True
+                self.board[row][col] = "." # 不合适，回溯
+        
+        return False # 空位置没法解决
+
+    def solveSudoku(self, board: List[List[str]]) -> None:
+        """
+        Do not return anything, modify board in-place instead.
+        """
+        if board is None or len(board) == 0:
+            return
+        self.board = board
+        self.isSolved()
+```
+
 Go：
 
 Javascript: