CODE 82: N-Queens

The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.

Given an integer n, return all distinct solutions to the n-queens puzzle.

Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space respectively.

For example,
There exist two distinct solutions to the 4-queens puzzle:

[ [".Q..",  // Solution 1  "...Q",  "Q...",  "..Q."], ["..Q.",  // Solution 2  "Q...",  "...Q",  ".Q.."]]

public ArrayList<String[]> solveNQueens(int n) {// Note: The Solution object is instantiated only once and is reused by// each test case.ArrayList<Integer> cols = new ArrayList<Integer>();ArrayList<String[]> result = createNQueens(n, cols, 0);return result;}public ArrayList<String[]> createNQueens(int n, ArrayList<Integer> cols,int col) {// Note: The Solution object is instantiated only once and is reused by// each test case.ArrayList<String[]> results = new ArrayList<String[]>();for (int i = 0; i < n; i++) {if (cols.contains((Integer) i)) {continue;} else if (!cols.isEmpty()) {int j;for (j = cols.size() - 1; j >= 0; j--) {if (Math.abs(i - cols.get(j)) == cols.size() - j) {break;}}if (j >= 0) {continue;}}cols.add((Integer) i);StringBuilder sb = new StringBuilder();for (int j = 0; j < n; j++) {if (j != i) {sb.append('.');} else {sb.append('Q');}}if (col == n - 1) {String[] strs = new String[n];strs[col] = sb.toString();results.add(strs);} else {ArrayList<String[]> nextResult = createNQueens(n, cols, col + 1);for (String[] strs : nextResult) {strs[col] = sb.toString();results.add(strs);}}cols.remove((Integer) i);}return results;}