Description
You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise).
给你一个 n x n 的二维矩阵 matrix 表示一张图片，将该图片顺时针旋转 90°。
You have to rotate the image in-place, which means you have to modify the input 2D matrix directly. DO NOT allocate another 2D matrix and do the rotation.
你必须原地完成旋转操作，也就是说，你必须直接修改原先的二维数组。不要另外分配二维数组完成旋转。
Examples
Example 1
Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [[7,4,1],[8,5,2],[9,6,3]]
Example 2
Input: matrix = [[5,1,9,11],[2,4,8,10],[13,3,6,7],[15,14,12,16]]
Output: [[15,13,2,5],[14,3,4,1],[12,6,8,9],[16,7,10,11]]
Example 3
Input: matrix = [[1]]
Output: [[1]]
Example 4
Input: matrix = [[1,2],[3,4]]
Output: [[3,1],[4,2]]
Constraints
Solution
一开始一直在想一步到位的骚操作，最后没想出来，一翻 discussion 后恍然大悟，一步做不出来，为什么不分成两步做呢？
顺时针 90° 旋转一张图片，需要以下两步（假设图片中心位于直角坐标原点）：
-
整张图片以 x 轴翻转
-
整张图片以主对角线（\(y = -x\)）进行翻转
代码如下：
class Solution {
fun rotate(matrix: Array<IntArray>): Unit {
flipVertical(matrix)
flipDiagonal(matrix)
}
private fun flipDiagonal(matrix: Array<IntArray>) {
for (i in matrix.indices) {
for (j in 0 until i) {
val t = matrix[i][j]
matrix[i][j] = matrix[j][i]
matrix[j][i] = t
}
}
}
private fun flipVertical(matrix: Array<IntArray>) {
var i = 0
var j = matrix.lastIndex
while (i < j) {
val t = matrix[i]
matrix[i] = matrix[j]
matrix[j] = t
i++
j--
}
}
}
