0048. Rotate Image

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

Description

You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise).

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:

**Input:** matrix = [[1,2,3],[4,5,6],[7,8,9]]
**Output:** [[7,4,1],[8,5,2],[9,6,3]]

Example 2:

**Input:** matrix = [[5,1,9,11],[2,4,8,10],[13,3,6,7],[15,14,12,16]]
**Output:** [[15,13,2,5],[14,3,4,1],[12,6,8,9],[16,7,10,11]]

Example 3:

**Input:** matrix = [[1]]
**Output:** [[1]]

Example 4:

**Input:** matrix = [[1,2],[3,4]]
**Output:** [[3,1],[4,2]]

Constraints:

• matrix.length == n

• matrix[i].length == n

• 1 <= n <= 20

• -1000 <= matrix[i][j] <= 1000

ac

it's all about imagination

class Solution {
    public void rotate(int[][] matrix) {
        // layer by layer, layer = (n+1) / 2 
        // 4 edges
        int mR = matrix.length;
        int mC = matrix.length;
        for (int layer = 0; layer < (mR+1)/2; layer++) {
            int len = mR - 2*layer - 1;
            for (int diff = 0; diff < len; diff++) {
                // top -> tmp
                int tmp = matrix[layer][layer+diff];
                // left -> top
                matrix[layer][layer+diff] = matrix[mR-1-layer-diff][layer];
                // buttom -> left
                matrix[mR-1-layer-diff][layer] = matrix[mR-1-layer][mC - 1 - layer - diff];
                // right -> buttom
                matrix[mR-1-layer][mC - 1 - layer - diff] = matrix[layer + diff][mC - 1 - layer];
                // tmp -> right
                matrix[layer + diff][mC - 1 - layer] = tmp;
            }
        }

    }
}