题解 | #螺旋矩阵#

import java.util.ArrayList;
public class Solution {
    public ArrayList<Integer> spiralOrder(int[][] matrix) {

        ArrayList<Integer> res = new ArrayList<Integer>();

        if(matrix.length == 0) return res;

        //分别记录上、下、左、右边界
        int up = 0;
        int bottom = matrix.length - 1;
        int left = 0;
        int right = matrix[0].length - 1;
        while(up < (matrix.length + 1)/2 && left <  (matrix[0].length + 1)/2){
                //顺时针填充

                //1.填充上边界
                for(int i = left ; i <= right ; i++){
                    res.add(matrix[up][i]);
                }
                //2.填充右边界
                for(int i = up + 1 ; i <= bottom ; i++){
                    res.add(matrix[i][right]);
                }
                //3.填充下边界
                for(int i = right - 1 ; up != bottom && i >= left  ; i--){
                    res.add(matrix[bottom][i]);
                }
                //4.填充左边界
                for(int i = bottom - 1 ; left != right && i >= up + 1   ; i--){
                    res.add(matrix[i][left]);
                }

                up++;
                bottom--;
                left++;
                right--;
        }

        return res;
        
    }
}