LeetCode - Valid Sudoku
Problem description
Determine if a 9x9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:
- Each row must contain the digits 1-9 without repetition.
- Each column must contain the digits 1-9 without repetition.
- Each of the 9 3x3 sub-boxes of the grid must contain the digits 1-9 without repetition.
The Sudoku board could be partially filled, where empty cells are filled with the character ‘.’.
Example 1:
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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:
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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.
Analysis
Just do the three rules. Here’s the code:
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public class ValidSudoku {
public boolean isValidSudoku(char[][] board) {
HashSet<Character> set1 = new HashSet<>();
HashSet<Character> set2 = new HashSet<>();
HashSet<Character> set3 = new HashSet<>();
for (int i = 0; i < 9; i++) {
set1.clear();
set2.clear();
for (int j = 0; j < 9; j++) {
char c1 = board[i][j];
if (c1 != '.' && set1.contains(c1)) {
return false;
}
if (c1 != '.') {
set1.add(c1);
}
char c2 = board[j][i];
if (c2 != '.' && set2.contains(c2)) {
return false;
}
if (c2 != '.') {
set2.add(c2);
}
}
}
for (int i = 0; i < 9; i += 3) {
for (int j = 0; j < 9; j += 3) {
set3.clear();
for (int k = i; k < i + 3; k++) {
for (int l = j; l < j + 3; l++) {
char c3 = board[k][l];
if (c3 != '.' && set3.contains(c3)) {
return false;
}
if (c3 != '.') {
set3.add(c3);
}
}
}
}
}
return true;
}
}