You are given an n x n 2D matrix representing an image.
Rotate the image by 90 degrees (clockwise).
Follow up:
Could you do this in-place?
5/17/19 店面
/*
The idea was firstly transpose the matrix and then flip it symmetrically.
For instance,
1 2 3
4 5 6
7 8 9
after transpose, it will be swap(matrix[i][j], matrix[j][i])
转置矩阵
1 4 7
2 5 8
3 6 9
Then flip the matrix horizontally. （水平翻转）
(swap(matrix[i][j], matrix[i][matrix.length-1-j])
7 4 1
8 5 2
9 6 3
*/
public void rotate(int[][] matrix) {
for (int i = 0; i < matrix.length; i++) {
for (int j = i; j < matrix[0].length; j++) {
int temp = 0;
temp = matrix[i][j];
matrix[i][j] = matrix[j][i];
matrix[j][i] = temp;
}
}
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix.length/2 ;j++) {
int temp = 0;
temp = matrix[i][j];
matrix[i][j] = matrix[i][matrix.length -1 -j];
matrix[i][matrix.length - 1 -j] = temp;
}
}
}
第二种思路
/*
* clockwise rotate
* first reverse up to down, then swap the symmetry
* 1 2 3 7 8 9 7 4 1
* 4 5 6 => 4 5 6 => 8 5 2
* 7 8 9 1 2 3 9 6 3
*/
/*
* anticlockwise rotate
* first reverse left to right, then swap the symmetry
* 1 2 3 3 2 1 3 6 9
* 4 5 6 => 6 5 4 => 2 5 8
* 7 8 9 9 8 7 1 4 7
*/
// Ref: https://leetcode.com/discuss/20589/a-common-method-to-rotate-the-image
// test case: [[1,2,3], [4,5,6],[7,8,9]]
public void rotate(int[][] matrix) {
if(matrix.length <=1) return;
int m = matrix.length, n = matrix[0].length;
swapRows(matrix);
// [[1,2,3,4],[5,6,7,8],[9,10,11,12],[13,14,15,16]]
for (int i=0; i < m; ++i) {
for (int j= i+1; j< n; ++j) {
int temp = matrix[i][j];
matrix[i][j] = matrix[j][i];
matrix[j][i] = temp;
}
}
}
public void swapRows(int[][] A) {
// swap rows
int len = A.length;
for (int i=0; i< len/2; i++) {
int[] tmp = A[i];
A[i] = A[len-1-i];
A[len-1-i] = tmp;
}
}
