Leetcode 51. N-Queens

public class Solution {
public static boolean canPlace(int row, int col, List<Integer> loc) {
// new Queen is at column of some other Queen which is not a valid position,
// make sure that there is no duplicate in the loc list
if (loc.contains(col)) return false;

// new Queen is at the diagonal of some other Queen,
// if |i - j| == |loc[i] - loc[j]| means tow Queens are at same diagonal
for (int i=0; i<loc.size(); i++)
if (Math.abs(i - loc.size()) == Math.abs(loc.get(i) - col))
return false;

return true;
}

public static void nQueens(int n, int row, List<Integer> loc, List<String> list, List<List<String>> ret) {
if (row == n) {
ret.add(new ArrayList<>(list));     // reaches end, add the current string list to the result
return;
}
else {
for (int col=0; col<n; col++) {      // search from column 0 to column n-1 for a given row
if (canPlace(row, col, loc)) {
loc.add(col);
char[] tmp = new char
;
Arrays.fill(tmp, '.');
tmp[col] = 'Q';
String tmpString = new String(tmp);
list.add(tmpString);

nQueens(n, row+1, loc, list, ret);      // move to the next row

list.remove(list.size()-1);
loc.remove(loc.size()-1);
}
}
}
}

public List<List<String>> solveNQueens(int n) {
List<List<String>> ret = new ArrayList<>();
nQueens(n, 0, new ArrayList<Integer>(), new ArrayList<String>(), ret);
//System.out.println(cnt);
return ret;
}
}