LeetCode - Rotate Image

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]
]

　　

解法1: 先以对角线为轴翻转得到其转置矩阵，再以中间竖轴翻转。

1  2  3　　　 　 1  4  7　　　　  7  4  1

4  5  6　-->　  2  5  8　 -->   8  5  2　　

7  8  9 　　　    3  6  9　　   　 9  6  3

 

class Solution {
    public void rotate(int[][] matrix) {
        if(matrix == null || matrix.length == 0 || matrix[0].length == 0){
            return;
        }
        int n = matrix.length;
        
        // along the left top to right bottom diagonal line, swap symmetrical pair
        for(int i = 0; i< n; i++){
            for(int j = i+1; j < n; j++){
                int temp = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = temp;
                
            }
        }
         // flip each row horizontally
        for(int i=0; i<n; i++){
            for(int j=0; j<n/2; j++){
                int temp = matrix[i][j];
                matrix[i][j] = matrix[i][n-1-j];
                matrix[i][n-1-j] = temp;
            }
        }
        
    }
}