54. Spiral Matrix 

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class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        int m = matrix.length, n = matrix[0].length;

        List<Integer> resultList = new ArrayList<>(m * n);

        int rowStartIndex = 0, rowEndIndex = m - 1, colStartIndex = 0, colEndIndex = n - 1;
        while (true) {
            // left -> right
            for (int j = colStartIndex; j <= colEndIndex; j++) {
                resultList.add(matrix[rowStartIndex][j]);
            }
            if (++rowStartIndex > rowEndIndex) {
                break;
            }
            // top -> bottom
            for (int i = rowStartIndex; i <= rowEndIndex; i++) {
                resultList.add(matrix[i][colEndIndex]);
            }
            if (--colEndIndex < colStartIndex) {
                break;
            }
            // right -> left
            for (int j = colEndIndex; j >= colStartIndex; j--) {
                resultList.add(matrix[rowEndIndex][j]);
            }
            if (--rowEndIndex < rowStartIndex) {
                break;
            }
            // bottom -> top
            for (int i = rowEndIndex; i >= rowStartIndex; i--) {
                resultList.add(matrix[i][colStartIndex]);
            }
            if (++colStartIndex > colEndIndex) {
                break;
            }
        }

        return resultList;
    }
}

注意四个边界判断，需要超出才跳出循环，等于时是可以打印的，因为定义的行列变量是可打印的边界，等于时表明该 行/列 还没有被打印。

References

54. Spiral Matrix
剑指 Offer 29. 顺时针打印矩阵