Create Leetcode famous problem: Sudoku Solver

LeetcodeProblems/37. Sudoku Solver

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+** 37. Sudoku Solver **
+
+# Approach 1:
+from typing import List
+
+class Solution:
+    def solveSudoku(self, board: List[List[str]]) -> bool:
+        def isValid(row: int, col: int, c: str) -> bool:
+            return c not in rows[row] and c not in cols[col] and c not in boxes[3 * (row // 3) + (col // 3)]
+
+        def placeNumber(row: int, col: int, c: str):
+            board[row][col] = c
+            rows[row].add(c)
+            cols[col].add(c)
+            boxes[3 * (row // 3) + (col // 3)].add(c)
+
+        def removeNumber(row: int, col: int, c: str):
+            board[row][col] = "."
+            rows[row].remove(c)
+            cols[col].remove(c)
+            boxes[3 * (row // 3) + (col // 3)].remove(c)
+
+        def backtrack(index: int) -> bool:
+            if index == len(empty_cells):
+                return True
+
+            row, col = empty_cells[index]
+            for c in "123456789":
+                if isValid(row, col, c):
+                    placeNumber(row, col, c)
+                    if backtrack(index + 1):
+                        return True
+                    removeNumber(row, col, c)
+            return False
+
+        rows = [set() for _ in range(9)]
+        cols = [set() for _ in range(9)]
+        boxes = [set() for _ in range(9)]
+
+        empty_cells = []
+
+        for r in range(9):
+            for c in range(9):
+                if board[r][c] == ".":
+                    empty_cells.append((r, c))
+                else:
+                    placeNumber(r, c, board[r][c])
+
+        empty_cells.sort(key=lambda cell: sum(1 for c in "123456789" if isValid(cell[0], cell[1], c)))
+
+        backtrack(0)
+
+# Approach 2:
+class Solution:
+    def solveSudoku(self, board: List[List[str]]) -> None:
+        n = 9
+        def isValid(row, col, ch):
+            row, col = int(row), int(col)
+
+            for i in range(9):
+
+                if board[i][col] == ch:
+                    return False
+                if board[row][i] == ch:
+                    return False
+
+                if board[3*(row//3) + i//3][3*(col//3) + i%3] == ch:
+                    return False
+
+            return True
+
+        def solve(row, col):
+            if row == n:
+                return True
+            if col == n:
+                return solve(row+1, 0)
+
+            if board[row][col] == ".":
+                for i in range(1, 10):
+                    if isValid(row, col, str(i)):
+                        board[row][col] = str(i)
+
+                        if solve(row, col + 1):
+                            return True
+                        else:
+                            board[row][col] = "."
+                return False
+            else:
+                return solve(row, col + 1)
+        solve(0, 0)