code

class Solution {
    List<List<String>> result = new ArrayList();
    Stack<String> path = new Stack();
    String[][] b;
    int n;
    boolean[] col;
    boolean[] ne;
    boolean[] nw;

    public List<List<String>> solveNQueens(int _n) {
        n = _n;
        col = new boolean[n];
        ne = new boolean[2*n];
        nw = new boolean[2*n];

        // 初始化棋盘
        b = new String[n][n];
        for(int i = 0; i < n; i++){
            for(int j = 0; j < n; j++){
                b[i][j] = ".";
            }
        }

        dfs(0);

        return result;
    }

    public void dfs(int depth){
        if(depth == n){
            path.clear();
            for(int i = 0; i < n; i++){
                StringBuilder sb = new StringBuilder();
                for(int j = 0; j < n; j++){
                    sb.append(b[i][j]);
                }
                path.push(sb.toString());
            }
            result.add(new ArrayList(path));
            return;
        }

        for(int i = 0; i < n; i++){
            if(!col[i] && !ne[i-depth+n] && !nw[i+depth]){
                col[i] = ne[i-depth+n] = nw[i+depth] = true;
                b[depth][i] = "Q";
                dfs(depth+1);
                b[depth][i] = ".";
                col[i] = ne[i-depth+n] = nw[i+depth] = false;
            }
        }
    }
}

视频讲解中的最核心的思路