• LeetCode 36. Valid Sudoku


     原题链接在这里:https://leetcode.com/problems/valid-sudoku/

    题目:

    Determine if a 9x9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:

    1. Each row must contain the digits 1-9 without repetition.
    2. Each column must contain the digits 1-9 without repetition.
    3. Each of the 9 3x3 sub-boxes of the grid must contain the digits 1-9 without repetition.


    A partially filled sudoku which is valid.

    The Sudoku board could be partially filled, where empty cells are filled with the character '.'.

    Example 1:

    Input:
    [
      ["5","3",".",".","7",".",".",".","."],
      ["6",".",".","1","9","5",".",".","."],
      [".","9","8",".",".",".",".","6","."],
      ["8",".",".",".","6",".",".",".","3"],
      ["4",".",".","8",".","3",".",".","1"],
      ["7",".",".",".","2",".",".",".","6"],
      [".","6",".",".",".",".","2","8","."],
      [".",".",".","4","1","9",".",".","5"],
      [".",".",".",".","8",".",".","7","9"]
    ]
    Output: true
    

    Example 2:

    Input:
    [
      ["8","3",".",".","7",".",".",".","."],
      ["6",".",".","1","9","5",".",".","."],
      [".","9","8",".",".",".",".","6","."],
      ["8",".",".",".","6",".",".",".","3"],
      ["4",".",".","8",".","3",".",".","1"],
      ["7",".",".",".","2",".",".",".","6"],
      [".","6",".",".",".",".","2","8","."],
      [".",".",".","4","1","9",".",".","5"],
      [".",".",".",".","8",".",".","7","9"]
    ]
    Output: false
    Explanation: Same as Example 1, except with the 5 in the top left corner being 
        modified to 8. Since there are two 8's in the top left 3x3 sub-box, it is invalid.
    

    Note:

    • A Sudoku board (partially filled) could be valid but is not necessarily solvable.
    • Only the filled cells need to be validated according to the mentioned rules.
    • The given board contain only digits 1-9 and the character '.'.
    • The given board size is always 9x9.

    Note:
    A valid Sudoku board (partially filled) is not necessarily solvable. Only the filled cells need to be validated.

    题解:

    先一行一行检查,再一列一列检查,这里注意外层for loop是j

    最后一个一个小方块来检查, for each box, have a count k within [0,8], have (i + k/3, j + k%3) as coordinate.

    Time Complexity: O(n^2). n = board.length. Space: O(1).

    AC Java:

     1 class Solution {
     2     public boolean isValidSudoku(char[][] board) {
     3         if(board == null || board.length != 9 || board[0].length != 9){
     4             return false;
     5         }
     6         
     7         for(int i = 0; i<9; i++){
     8             HashSet<Character> hs = new HashSet<>();
     9             for(int j = 0; j<9; j++){
    10                 if(board[i][j] != '.'){
    11                     if(!Character.isDigit(board[i][j]) || !hs.add(board[i][j])){
    12                         return false;
    13                     }
    14                 }
    15             }
    16         }
    17         
    18         for(int j = 0; j<9; j++){
    19             HashSet<Character> hs = new HashSet<>();
    20             for(int i = 0; i<9; i++){
    21                 if(board[i][j] != '.'){
    22                     if(!Character.isDigit(board[i][j]) || !hs.add(board[i][j])){
    23                         return false;
    24                     }
    25                 }
    26             }
    27         }
    28         
    29         for(int i = 0; i<9; i+=3){
    30             for(int j = 0; j<9; j+=3){
    31                 HashSet<Character> hs = new HashSet<>();
    32                 for(int k = 0; k<9; k++){
    33                     if(board[i + k/3][j + k%3] != '.'){
    34                         if(!hs.add(board[i + k/3][j + k%3])){
    35                             return false;
    36                         }
    37                     }
    38                 }
    39             }
    40         }
    41         
    42         return true;
    43     }
    44 }

    跟上Sudoku Solver.

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  • 原文地址:https://www.cnblogs.com/Dylan-Java-NYC/p/4825064.html
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