Leetcode: https://leetcode.com/problems/rotate-image/description/

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]

• (x, y), the affected point is
  (y, n – x – 1), (n – x – 1, n – y – 1), (n – y – 1, x),  “n” is the size of the matrix.
  The rotate function=
  (x, y) -> (y, n – x – 1) -> (n – x – 1, n – y – 1) -> (n – y – 1, x) -> (x, y).
• For the i th line, we start from (i, i) ( !! in for loop  j = i ) , and ends at (i, n – 2 – i).  
• We only need process half of them. n/2

Reference solution:

• http://www.cnblogs.com/grandyang/p/4389572.html
• http://www.lifeincode.net/programming/leetcode-rotate-image-java/

class Solution {
    public void rotate(int[][] matrix) {
        int n = matrix.length;

        for(int i = 0; i < n / 2; i++){
            int colEnd = n-2-i;

            // j = i
            for (int j = i; j < colEnd+1; j++){
                int temp = matrix[i][j];
                matrix[i][j] = matrix[n-j-1][i];
                matrix[n-j-1][i] = matrix[n-i-1][n-j-1];
                matrix[n-i-1][n-j-1] = matrix[j][n-i-1];
                matrix[j][n-i-1] = temp;
            }
        }

    }
}