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0036-valid-sudoku.java

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class Solution {

public boolean isValidSudoku(char[][] board) {

//neetcode solution, slightly modified

//a set of the characters that we have already come across (excluding '.' which denotes an empty space)

Set<Character> rowSet = null;

Set<Character> colSet = null;

for (int i = 0; i < 9; i++) {

//reinitialize the sets so we don't carry over found characters from the previous run

rowSet = new HashSet<>();

colSet = new HashSet<>();

for (int j = 0; j < 9; j++) {

char r = board[i][j];

char c = board[j][i];

if (r != '.'){

if (rowSet.contains(r)){

return false;

} else {

rowSet.add(r);

}

}

if (c != '.'){

if (colSet.contains(c)){

return false;

} else {

colSet.add(c);

}

}

}

}

//block

//loop controls advance by 3 each time to jump through the boxes

for (int i = 0; i < 9; i = i + 3) {

for (int j = 0; j < 9; j = j + 3) {

//checkBlock will return true if valid

if (!checkBlock(i, j, board)) {

return false;

}

}

}

//passed all tests, therefore valid board

return true;

}

public boolean checkBlock(int idxI, int idxJ, char[][] board) {

Set<Character> blockSet = new HashSet<>();

//if idxI = 3 and indJ = 0

//rows = 6 and cols = 3

int rows = idxI + 3;

int cols = idxJ + 3;

//and because i initializes to idxI but only goes to rows, we loop 3 times (once for each row)

for (int i = idxI; i < rows; i++) {

//same for columns

for (int j = idxJ; j < cols; j++) {

if (board[i][j] == '.') {

continue;

}

if (blockSet.contains(board[i][j])) {

return false;

}

blockSet.add(board[i][j]);

}

}

return true;

}

}