算法:顺时针打印矩阵

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<>();

        int row = matrix.length, col = matrix[0].length;
        ArrayList<Integer> res = new ArrayList<>(row * col + 1);
        int left = 0, up = 0, down = row - 1, right = col - 1;
        while (left <= right && up <= down) {
            // left -> right
            for (int i = left; i <= right; i++) res.add(matrix[up][i]);
            // up -> down
            for (int i = up + 1; i <= down; ++i) res.add(matrix[i][right]);
            // right -> left
            if (up != down) {
                for (int i = right - 1; i >= left; --i) res.add(matrix[down][i]);
            }
            // down -> up
            if (left != right) {
                for (int i = down - 1; i > up; --i) res.add(matrix[i][left]);
            }
            left++;
            up++;
            down--;
            right--;
        }
        return res;
    }
}