题解 | #顺时针打印矩阵#

import java.util.*;
import java.util.ArrayList;
public class Solution {
    public ArrayList<Integer> printMatrix(int [][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
            return new ArrayList<>();
        }
        List<Integer> list = new ArrayList<>(matrix.length * matrix[0].length);

        // 最左      最右                  最上     最下
        int a = -1, b = matrix[0].length, c = -1, d = matrix.length;

        // 右1 下2 左3 上4
        int dir = 1;

        while (list.size() < matrix.length * matrix[0].length) {
            if (dir == 1 && list.size() < matrix.length * matrix[0].length) {
                c ++;
                for (int i = a + 1; i <= b - 1; i ++) {
                    list.add(matrix[c][i]);
                }
                dir = 2;
            }
            if (dir == 2 && list.size() < matrix.length * matrix[0].length) {
                b --;
                for (int i = c + 1; i <= d - 1; i ++) {
                    list.add(matrix[i][b]);
                }
                dir = 3;
            }
            if (dir == 3 && list.size() < matrix.length * matrix[0].length) {
                d --;
                for (int i = b - 1; i >= a + 1; i --) {
                    list.add(matrix[d][i]);
                }
                dir = 4;
            }
            if (dir == 4 && list.size() < matrix.length * matrix[0].length) {
                a ++;
                for (int i = d - 1; i >= c + 1; i --) {
                    list.add(matrix[i][a]);
                }
                dir = 1;
            }
        }

        return (ArrayList<Integer>) list;
    }
}