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[n];                    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;    }}