矩阵（一般是二维数组Vector）操作的题目，一般来说不带有太多算法思想，纯粹就是二维数组下标的操作.
Don't use the matrixp[x]p[y] in the code, it is easy to make the mistake. The first coordinate is usually thought of as the row number, which is in fact the y value. We should wirht matrix[r][c] to make life more easy. r = row, and c = column.