LeetCode 48. Rotate Image(java)

You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Note:

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.

Example 1:

Given input matrix =
[
[1,2,3],
[4,5,6],
[7,8,9]
],

rotate the input matrix in-place such that it becomes:
[
[7,4,1],
[8,5,2],
[9,6,3]
]

Example 2:

Given input matrix =
[
[ 5, 1, 9,11],
[ 2, 4, 8,10],
[13, 3, 6, 7],
[15,14,12,16]
],

rotate the input matrix in-place such that it becomes:
[
[15,13, 2, 5],
[14, 3, 4, 1],
[12, 6, 8, 9],
[16, 7,10,11]
]

思路：step1: 先将矩阵按列两两对换（reverse一下列的顺序），step2: 再按照右对角线对称，把元素交换即可。

例：
[1,2,3]      [3,2,1]      [7,4,1]
[4,5,6]  ->  [6,5,4]  ->  [8,5,2]
[7,8,9]      [9,8,7]      [9,6,3]

代码:

note: 注意第二步对换的时候，只需要遍历左上半的矩阵即可，如果全部遍历，就又换回来了！另外，如果写辅助函数swap，而传的参数不是数组类型，只是传两个值的话，在swap执行完，是不会改变原数组的内容的！

public void rotate(int[][] matrix) {
if (matrix.length <= 1) return;
int n = matrix.length - 1;
for (int i = 0; i <= n; i++) {
reverse(matrix[i]);
}
int m = n;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
int temp = matrix[i][j];
matrix[i][j] = matrix[n - j][n - i];
matrix[n - j][n - i] = temp;
}
m--;
}
return;
}
public void reverse(int[] line) {
int begin = 0, end = line.length - 1;
while (begin < end) {
int temp = line[begin];
line[begin++] = line[end];
line[end--] = temp;
}
return;
}