解决思路

模拟

绘制螺旋轨迹路径，我们发现当路径超出界限或者进入之前访问过的单元格时，会顺时针旋转方向。

public List<Integer> spiralOrder(int[][] matrix){
       List ans = new ArrayList();
       if(matrix.length == 0) return ans;
       int R = matrix.length,C =matrix[0].length;
       boolean[][] seen = new boolean[R][C];
       int[][] d = new int[][]{{0,1},{1,0},{0,-1},{-1,0}};
       int r = 0,c =0,di = 0;
       for(int i =0;i<R*C;i++){
           ans.add(matrix[r][c]);
           seen[r][c] = true;
           int cr = r + d[di][0];
           int cc = c + d[di][1];
           if(0<=cr && cr<R && 0<=cc && cc<C &&!seen[cr][cc]){
               r = cr;
               c = cc;
           }else{
               di = (di+1)%4;
               r += d[di][0];
               c += d[di][1];
           }
       }
       return ans;
   }