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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0001_0100.s0051_n_queens;

// #Hard #Top_100_Liked_Questions #Array #Backtracking #Big_O_Time_O(N!)_Space_O(N)
// #2024_11_11_Time_1_ms_(99.77%)_Space_44.8_MB_(61.16%)

import java.util.ArrayList;
import java.util.List;

public class Solution {
    public List> solveNQueens(int n) {
        char[][] board = new char[n][n];
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                board[i][j] = '.';
            }
        }
        List> res = new ArrayList<>();
        int[] leftRow = new int[n];
        int[] upperDiagonal = new int[2 * n - 1];
        int[] lowerDiagonal = new int[2 * n - 1];
        solve(0, board, res, leftRow, lowerDiagonal, upperDiagonal);
        return res;
    }

    void solve(
            int col,
            char[][] board,
            List> res,
            int[] leftRow,
            int[] lowerDiagonal,
            int[] upperDiagonal) {
        if (col == board.length) {
            res.add(construct(board));
            return;
        }
        for (int row = 0; row < board.length; row++) {
            if (leftRow[row] == 0
                    && lowerDiagonal[row + col] == 0
                    && upperDiagonal[board.length - 1 + col - row] == 0) {
                board[row][col] = 'Q';
                leftRow[row] = 1;
                lowerDiagonal[row + col] = 1;
                upperDiagonal[board.length - 1 + col - row] = 1;
                solve(col + 1, board, res, leftRow, lowerDiagonal, upperDiagonal);
                board[row][col] = '.';
                leftRow[row] = 0;
                lowerDiagonal[row + col] = 0;
                upperDiagonal[board.length - 1 + col - row] = 0;
            }
        }
    }

    List construct(char[][] board) {
        List res = new ArrayList<>();
        for (char[] chars : board) {
            String s = new String(chars);
            res.add(s);
        }
        return res;
    }
}