Leetcode|棋盘问题|51. N 皇后

回溯法

代码语言：javascript复制

class Solution {
private:
    vector<vector<string>> solution;
    vector<string> board;
    int size = 0;
public:
    bool notValid(int row, int col) {
        for (int i = row - 1;i >= 0;i--) 
            if (board[i][col] == 'Q') return true;
        for (int i = row - 1, j = col - 1;i >= 0 && j >= 0;i--, j--)
            if (board[i][j] == 'Q') return true;
        for (int i = row - 1, j = col   1;i >= 0 && j < size;i--, j  )
            if (board[i][j] == 'Q') return true;
        return false;
    }
    void backtrack(int row) {
        /**1.路径(row)；2.选择列表(board-col)**/
        if (row == size) {
            solution.emplace_back(board);
            return;
        }
        for (int col = 0;col < size;col  ) {
            if (notValid(row, col)) continue;  // 剪枝不合法选择
            board[row][col] = 'Q';  // 做选择
            backtrack(row   1);
            board[row][col] = '.';  // 撤销选择
        }
    }
    vector<vector<string>> solveNQueens(int n) {
        size = n;
        board = vector<string> (n, string(n, '.'));
        backtrack(0);
        return solution;
    }
};

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