【矩阵】54.螺旋矩阵（顺时针打印矩形元素）

题目

class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        int m = matrix.length, n = matrix[0].length;
        int leftUpM = 0, leftUpN = 0, rightDownM = m - 1, rightDownN = n - 1;
        List<Integer> res = new ArrayList<>();
        while (leftUpM <= rightDownM && leftUpN <= rightDownN) {
            for (int i = leftUpN; i <= rightDownN; ++i) { // 向右
                res.add(matrix[leftUpM][i]);
            }
            for (int i = leftUpM + 1; i < rightDownM; ++i) { // 向下
            	res.add(matrix[i][rightDownN]);
            }
            for (int i = rightDownN; i >= leftUpN && leftUpM < rightDownM; --i) { // 向左
                res.add(matrix[rightDownM][i]);
            }
            for (int i = rightDownM - 1; i > leftUpM && leftUpN < rightDownN; --i) { // 向上
                res.add(matrix[i][leftUpN]);
            }
            ++leftUpM;
            ++leftUpN;
            --rightDownM;
            --rightDownN;
        }

        return res;
    }
}