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public class NQueensII {
public int totalNQueens(int n) {
List<Integer> result = new ArrayList<Integer>();
result.add(0);
if (n < 1) {
return 0;
}
helper(result, new ArrayList<String>(), 0, n);
return result.get(0);
}
private void helper(List<Integer> result, List<String> crt, int row, int n) {
if (row == n) {
result.add(0, result.get(0) + 1);
return;
}
for (int i = 0; i < n; i++) {
if (isValid(crt, row, i, n)) {
String s = lineGenerator(i, n);
crt.add(s);
helper(result, crt, row + 1, n);
crt.remove(crt.size() - 1);
}
}
}
private boolean isValid(List<String> crt, int x, int y, int n) {
for (int i = 0; i < x; i++) {
int offSet = x - i;
if (crt.get(i).charAt(y) == 'Q') {
return false;
}
if (y - offSet >= 0 && crt.get(i).charAt(y - offSet) == 'Q') {
return false;
}
if (y + offSet < n && crt.get(i).charAt(y + offSet) == 'Q') {
return false;
}
}
return true;
}
private String lineGenerator(int x, int n) {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < n; i++) {
if (i == x) {
sb.append('Q');
} else {
sb.append('.');
}
}
return sb.toString();
}
}
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