48. Rotate Image - Medium

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]
]

 

把正方形的四条边分为四组，每组的n个数字位置互相占据，外层交换完成后进入内层

time: O(N^2), space: O(1)

class Solution {
    public void rotate(int[][] matrix) {
        int lrow = 0, lcol = 0, rrow = matrix.length - 1, rcol = matrix[0].length - 1;
        while(lrow < rrow && lcol < rcol) {
             for(int i = 0; i < rcol - lcol; i++) {
                 int tmp = matrix[lrow][lcol + i];
                 matrix[lrow][lcol + i] = matrix[rrow - i][lcol];
                 matrix[rrow - i][lcol] = matrix[rrow][rcol - i];
                 matrix[rrow][rcol - i] = matrix[lrow + i][rcol];
                 matrix[lrow + i][rcol] = tmp;
             }
            lrow++;lcol++;rrow--;rcol--;
        }
    }
}