51. N 皇后

• 51. N 皇后
• • 方法一：普通回溯

51. N 皇后

方法一：普通回溯

class Solution {
    private int n;
    private List<List<String>> res;
    private List<String> board;
    
    public List<List<String>> solveNQueens(int n) {
        this.n = n;
        res = new ArrayList<>();
        board = new ArrayList<>();
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; ++i) {
            sb.append('.');
        }
        for (int i = 0; i < n; ++i) {
            board.add(sb.toString());
        }
        backtrack(0);
        return res;
    }

    private void backtrack(int row) {
        if (row == n) {
            res.add(new ArrayList<>(board));
            return;
        }
        for (int col = 0; col < n; ++col) {
            //terminate
            if (!isValid(row, col)) {
                continue;
            }
            //process
            setChar(row, col, 'Q');
            //recursive
            backtrack(row + 1);
            //reverse
            setChar(row, col, '.');
        }
    }
    
    private boolean isValid(int row, int col) {
        for (int i = 0; i < row; ++i) {
            if (board.get(i).charAt(col) == 'Q'){
                return false;
            }
        }
        for (int i = row - 1, j = col - 1; i >=0 && j >= 0; --i, --j) {
            if (board.get(i).charAt(j) == 'Q') {
                return false;
            }
        }
        for (int i = row - 1, j = col + 1; i >= 0 && j < n; --i, ++j) {
            if (board.get(i).charAt(j) == 'Q') {
                return false;
            }
        }
        return true;
    }
    
    private void setChar(int row, int col, char c) {
        StringBuilder sb = new StringBuilder(board.get(row));
        sb.setCharAt(col, c);
        board.set(row, sb.toString());
    }
}