[LeetCode]51 N-Queens

public class Solution {
public List<String[]> solveNQueens(int n) {

List<String[]> result = new ArrayList<>();
int[] qcols = new int
;
solve(n, 0, qcols, result);
return result;

}

private void solve(int n, // number of queen
int row, // current row iteration, [0 -> n-1]
int[] qcols, // describing queens positions
List<String[]> result)
{
if (row == n)
{
// all [0, n-1] has fulfilled, a solution found.
result.add(toBoard(qcols));
return;
}

// We are on row
// try to put Q in each columns, and check
for (int i = 0 ; i < n ; i ++)
{
qcols[row] = i;
boolean valid = checkBoard(qcols, row);
if (valid)
{
// Enter next iteration
solve(n, row + 1, qcols, result);
}
}
}

// 此方法检查新增的Q（row）是否符合已有的棋盘
// 因为一行只保存一个Q的位置，所以行一定符合
// 检查列：qcols[i] != qcols[row]
// 检查对角线 abs(i - row) != abs(qcols[i] - qcols[row])
// Check new Q(on row) can fit existing board.
private boolean checkBoard(int[] qcols, int row)
{
for (int i = 0 ; i < row ; i ++)
{
if ((qcols[i] == qcols[row]) ||
Math.abs(i - row) == Math.abs(qcols[i] - qcols[row]))
return false;
}
return true;
}

// Build the board.
private String[] toBoard(int[] qcols)
{
int n = qcols.length;
String[] b = new String
;
for (int i = 0 ; i < n ; i ++)
{
int qcol = qcols[i];
StringBuilder sb = new StringBuilder();
for (int j = 0 ; j < qcol ; j ++)
sb.append(".");
sb.append("Q");
for (int j = qcol + 1 ; j < n ; j ++)
sb.append(".");
b[i] = sb.toString();
}
return b;
}
}