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51.java
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51.java
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__________________________________________________________________________________________________
sample 1 ms submission
class Solution {
public List<List<String>> solveNQueens(int n) {
// first generate a board filled with '.'
char[][] board = new char[n][n];
for (char[] line : board) {
Arrays.fill(line, '.');
}
List<List<String>> solutions = new ArrayList<List<String>>();
dfs(0, n, new boolean[2 * n], new boolean[2 * n - 1], new boolean[2 * n - 1], solutions, board);
return solutions;
}
private void dfs(int row, int n, boolean[] cols, boolean[] diag1, boolean[] diag2,
List<List<String>> solutions, char[][] board) {
if (row == n) {
// convert the board to List<String> and add to answer set
List<String> res = new ArrayList<String>();
for (char[] line : board) {
res.add(String.valueOf(line));
}
solutions.add(res);
return;
}
for (int i = 0; i < n; i++) {
if (cols[i] || diag1[row - i + n - 1] || diag2[i + row]) continue;
board[row][i] = 'Q';
cols[i] = true;
diag1[row - i + n - 1] = true;
diag2[i + row] = true;
dfs(row + 1, n, cols, diag1, diag2, solutions, board);
cols[i] = false;
diag1[row - i + n - 1] = false;
diag2[i + row] = false;
board[row][i] = '.';
}
}
}
__________________________________________________________________________________________________
sample 37472 kb submission
class Solution {
public List<List<String>> solveNQueens(int n) {
int[] rows = new int[n];
List<List<String>> solutions = new ArrayList<>();
solve(rows, solutions, 0);
return solutions;
}
private void solve(int[] rows, List<List<String>> solutions, int cur) {
if (cur == rows.length) {
solutions.add(encode(rows));
return;
}
for (int i = 0; i < rows.length; i++) {
boolean good = true;
for (int j = 0; j < cur; j++) {
if (rows[j] == i) {
good = false;
break;
}
int d = cur - j;
if (rows[j] == i + d || rows[j] == i - d) {
good = false;
break;
}
}
if (good) {
rows[cur] = i;
solve(rows, solutions, cur + 1);
}
}
}
private List<String> encode(int[] rows) {
List<String> output = new ArrayList<>();
for (int row : rows) {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < rows.length; i++) {
sb.append(i == row ? 'Q' : '.');
}
output.add(sb.toString());
}
return output;
}
}
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