g0001_0100.s0051_n_queens.Solution.cpp Maven / Gradle / Ivy

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104 LeetCode algorithm problem solutions

There is a newer version: 1.8

// #Hard #Top_100_Liked_Questions #Array #Backtracking #Big_O_Time_O(N!)_Space_O(N)
// #2024_05_25_Time_3_ms_(80.52%)_Space_11.3_MB_(29.15%)

#include 
#include 
#include 

class Solution {
public:
    std::vector> solveNQueens(int n) {
        std::vector pos(n + 2 * n - 1 + 2 * n - 1, false);
        std::vector pos2(n, 0);
        std::vector> ans;
        helper(n, 0, pos, pos2, ans);
        return ans;
    }

private:
    void helper(int n, int row, std::vector& pos, std::vector& pos2, std::vector>& ans) {
        if (row == n) {
            construct(n, pos2, ans);
            return;
        }
        for (int i = 0; i < n; ++i) {
            int 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;
        }
    }

    void construct(int n, std::vector& pos, std::vector>& ans) {
        std::vector sol;
        for (int r = 0; r < n; ++r) {
            std::string queenRow(n, '.');
            queenRow[pos[r]] = 'Q';
            sol.push_back(queenRow);
        }
        ans.push_back(sol);
    }
};