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:

class Solution {
    public boolean isValidSudoku(char[][] board) {
        if(board == null || board.length != 9 || board[0].length != 9){
            return false;
        }
        
        for(int i = 0; i<9; i++){
            HashSet<Character> hs = new HashSet<>();
            for(int j = 0; j<9; j++){
                if(board[i][j] != '.'){
                    if(!Character.isDigit(board[i][j]) || !hs.add(board[i][j])){
                        return false;
                    }
                }
            }
        }
        
        for(int j = 0; j<9; j++){
            HashSet<Character> hs = new HashSet<>();
            for(int i = 0; i<9; i++){
                if(board[i][j] != '.'){
                    if(!Character.isDigit(board[i][j]) || !hs.add(board[i][j])){
                        return false;
                    }
                }
            }
        }
        
        for(int i = 0; i<9; i+=3){
            for(int j = 0; j<9; j+=3){
                HashSet<Character> hs = new HashSet<>();
                for(int k = 0; k<9; k++){
                    if(board[i + k/3][j + k%3] != '.'){
                        if(!hs.add(board[i + k/3][j + k%3])){
                            return false;
                        }
                    }
                }
            }
        }
        
        return true;
    }
}

跟上Sudoku Solver.