# My clean java code in 54 lines

• public class Solution {
Map<Integer, Set<Character>> rows = new HashMap<Integer, Set<Character>>(),
columns = new HashMap<Integer, Set<Character>>(),
cubes = new HashMap<Integer, Set<Character>>();
Set<Character> init_set = new HashSet<Character>(Arrays.asList(new Character[]{'1','2','3','4','5','6','7','8','9'}));

``````public void solveSudoku(char[][] board) {
for (int i = 0; i < 9; ++i) {
Set<Character> row = new HashSet<Character>(init_set),
column = new HashSet<Character>(init_set),
cube = new HashSet<Character>(init_set);
for (int j = 0; j < 9; ++j) {
if (board[i][j] != '.')
row.remove(board[i][j]);
if (board[j][i] != '.')
column.remove(board[j][i]);
int a = 3 * (i / 3), b = 3 * (i % 3);
int c = j / 3, d = j % 3;
if (board[a + c][b + d] != '.')
cube.remove(board[a + c][b + d]);
}
rows.put(i, row);
columns.put(i, column);
cubes.put(i, cube);
}
dfs(board, 0, 0);
}

public boolean dfs(char[][] board, int i, int j) {
if (i == 9)
return true;
int next_i = i, next_j = j;
if (j == 8) {
next_i = i + 1; next_j = 0;
}
else {
next_j += 1;
}
if (board[i][j] != '.') {
return dfs(board, next_i, next_j);
}
else {
Set<Character> r = rows.get(i), c = columns.get(j),
cube = cubes.get(3 * (i / 3) + j / 3);
Set<Character> tmp = new HashSet<Character>(init_set);
tmp.retainAll(r); tmp.retainAll(c); tmp.retainAll(cube);
for (Character candi: tmp) {
board[i][j] = candi; r.remove(candi); c.remove(candi); cube.remove(candi);
if (dfs(board, next_i, next_j))
return true;
}
return false;
}
}
``````

}

• What is the runtime?
And how is the runtime is measured anyway? It is something that is dependent on underlying hardware and OS and jvm etc.

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