LeetCode-in-All
51. N-Queens
Hard
The n-queens puzzle is the problem of placing n queens on an n x n chessboard such that no two queens attack each other.
Given an integer n, return all distinct solutions to the n-queens puzzle. You may return the answer in any order.
Each solution contains a distinct board configuration of the n-queens’ placement, where 'Q' and '.' both indicate a queen and an empty space, respectively.
Example 1:
Input: n = 4
Output: [[“.Q..”,”…Q”,”Q…”,”..Q.”],[”..Q.”,”Q…”,”…Q”,”.Q..”]]
Explanation: There exist two distinct solutions to the 4-queens puzzle as shown above
Example 2:
Input: n = 1
Output: [[“Q”]]
Constraints:
1 <= n <= 9
Solution
class Solution {
fun solveNQueens(n: Int): List<List<String>> {
val pos = BooleanArray(n + 2 * n - 1 + 2 * n - 1)
val pos2 = IntArray(n)
val ans: MutableList<List<String>> = ArrayList()
helper(n, 0, pos, pos2, ans)
return ans
}
private fun helper(n: Int, row: Int, pos: BooleanArray, pos2: IntArray, ans: MutableList<List<String>>) {
if (row == n) {
construct(n, pos2, ans)
return
}
for (i in 0 until n) {
val index = n + 2 * n - 1 + n - 1 + i - row
if (pos[i] || pos[n + i + row] || pos[index]) {
continue
}
pos[i] = true
pos[n + i + row] = true
pos[index] = true
pos2[row] = i
helper(n, row + 1, pos, pos2, ans)
pos[i] = false
pos[n + i + row] = false
pos[index] = false
}
}
private fun construct(n: Int, pos: IntArray, ans: MutableList<List<String>>) {
val sol: MutableList<String> = ArrayList()
for (r in 0 until n) {
val queenRow = CharArray(n)
queenRow.fill('.')
queenRow[pos[r]] = 'Q'
sol.add(String(queenRow))
}
ans.add(sol)
}
}