Just remember this formula and you are all set:

i' = j;

j' = n-1-i;

Code:

```
rotate(int[][] matrix){
//if null do nothing Obviously!!
if(matrix==null || matrix.length==0) return;
//store matrix dimension
int n = matrix.length();
int counter = n/2; //need to run the loop this many times
for(int c =0; c<counter;c++){
for(int i=c,j=c;j<n-1-c;j++){
int temp = matrix[i][j];
//Remember formula : i' = j , j' = n-1-i
int i2 = j;
int j2 = n-1-i;
int i3 = j2;
int j3 = n-1-i2;
int i4 = j3;
int j4 = n-1-i3;
// Good to go!!
matrix[i][j] = matrix[i4][j4];
matrix[i4][j4] = matrix[i3][j3];
matrix[i3][j3] = matrix[i2][j2];
matrix[i2][j2] = temp; // matrix[i][j]
}
}
}
```