原题链接: https://oj.leetcode.com/problems/rotate-image/

/*
 * [1, 1, 1, 1, 1]
 * [1, 2, 2, 2, 1]
 * [1, 2, 3, 2, 1]
 * [1, 2, 2, 2, 1]
 * [1, 1, 1, 1, 1]
 * 
 * Like shown above, take the 2-dim array as it has some layers wrapping each other, layer 1 wraps layer 2, layer 2 wraps layer 3......
 * For each layer, we can divide it into four blocks : top row, right col, bottom row, left col.
 * What we have to do is : move top row to right col, right col to bottom row, bottom row to left col, left col to top row.
 * And, we do this for times of numbers of layers(matrix's length / 2).  
 */
public void rotate(int[][] matrix) {
    if (matrix == null || matrix.length == 0 || matrix[0].length == 0) return;

    int matrixSize = matrix.length;
    int layerNum = matrixSize / 2;

    for (int i = 0; i < layerNum; i++) {
        for (int low = i, high = matrixSize - i - 1; low < high; low++) {
            int temp = matrix[i][low];

            // move left col to top row
            matrix[i][low] = matrix[matrixSize - low - 1][i];

            // move bottom row to left col
            matrix[matrixSize - low - 1][i] = matrix[high][matrixSize - low - 1];

            // move right col to bottom row
            matrix[high][matrixSize - low - 1] = matrix[low][high];

            // move top row to right col
            matrix[low][high] = temp; 
        }
    }
}