Solution.java

class Solution {

    //Time Complexity: O(n^2)
    //Space Complexity: O(1)
    public void rotate(int[][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
            return;
        }
        int n = matrix.length;
        int start = 0, end = n - 1;
        //首尾行交换
        while (start < end) {
            int[] temp = matrix[start];
            matrix[start] = matrix[end];
            matrix[end] = temp;
            start ++;
            end --;
        }
        //以主对角线为轴交换
        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;
            }
        }
    }
}