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, the point is to find the upper left coordiante of each box. 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.