30天【代码随想录算法训练营34期】第七章 回溯算法part06 （● 332.重新安排行程 ● 51. N皇后 ● 37. 解数独 ● 总结 ）

332.重新安排行程
木有看懂，没视频所以也没看懂

51. N皇后
自己写出来还是有难度的

class Solution:
    def solveNQueens(self, n: int) -> List[List[str]]:
        result = []  # 存储最终结果的二维字符串数组

        chessboard = ['.' * n for _ in range(n)]  # 初始化棋盘
        self.backtracking(n, 0, chessboard, result)  # 回溯求解
        return [[''.join(row) for row in solution] for solution in result]  # 返回结果集

    def backtracking(self, n: int, row: int, chessboard: List[str], result: List[List[str]]) -> None:
        if row == n:
            result.append(chessboard[:])  # 棋盘填满，将当前解加入结果集
            return

        for col in range(n):
            if self.isValid(row, col, chessboard):
                chessboard[row] = chessboard[row][:col] + 'Q' + chessboard[row][col+1:]  # 放置皇后
                self.backtracking(n, row + 1, chessboard, result)  # 递归到下一行
                chessboard[row] = chessboard[row][:col] + '.' + chessboard[row][col+1:]  # 回溯，撤销当前位置的皇后

    def isValid(self, row: int, col: int, chessboard: List[str]) -> bool:
        # 检查列
        for i in range(row):
            if chessboard[i][col] == 'Q':
                return False  # 当前列已经存在皇后，不合法

        # 检查 45 度角是否有皇后
        i, j = row - 1, col - 1
        while i >= 0 and j >= 0:
            if chessboard[i][j] == 'Q':
                return False  # 左上方向已经存在皇后，不合法
            i -= 1
            j -= 1

        # 检查 135 度角是否有皇后
        i, j = row - 1, col + 1
        while i >= 0 and j < len(chessboard):
            if chessboard[i][j] == 'Q':
                return False  # 右上方向已经存在皇后，不合法
            i -= 1
            j += 1

        return True  # 当前位置合法

37. 解数独
很有道理，但是一脱离模板就不会走路了

class Solution:
    def solveSudoku(self, board: List[List[str]]) -> None:
        self.backtracking(board)
    
    def backtracking(self, board):
        for i in range(len(board)):
            for j in range(len(board[0])):
                if board[i][j] != '.':
                    continue
                for k in range(1, 10):
                    if self.isValid(i, j, k, board):
                        board[i][j] = str(k)
                        if self.backtracking(board):
                            return True
                        board[i][j] = '.'
                return False
        return True

        
    def isValid(self, row, col, num, board):
        for i in range(9):
            if board[row][i] == str(num):
                return False
            if board[i][col] == str(num):
                return False

        start_col = (col // 3)*3
        start_row = (row // 3)*3

        for i in range(start_row, start_row+3):
            for j in range(start_col, start_col+3):
                if board[i][j] == str(num):
                    return False
        return True

总结