54-螺旋矩阵

public List<Integer> spiralOrder(int[][] matrix) {
    List<Integer> res = new ArrayList<>();
    if(matrix == null || matrix.length == 0 || matrix[0].length == 0) return res;
    int[] dx = {0, 1, 0, -1};	//方向指引
    int[] dy = {1, 0, -1, 0};
    int x = 0, y = -1, t = 0;	//t代表第几次方向，x,y初值是因为一开始往右走
    boolean[][] isPass = new boolean[matrix.length][matrix[0].length];
    for(int i = 0; i < matrix.length * matrix[0].length; i++){
        if(!check(matrix, x + dx[t], y + dy[t], isPass)){
            //转向
            t = (t + 1) % 4;
        }
        x += dx[t];
        y += dy[t];
        res.add(matrix[x][y]);
        isPass[x][y] = true;
    }
    return res;
}

public boolean check(int[][] matrix, int x, int y, boolean[][] isPass){
    return x >= 0 && x < matrix.length && y >= 0 && y < matrix[0].length && !isPass[x][y];
}