leetcode解题之48. Rotate Image java版（旋转图像）

48. Rotate Image

You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Follow up:
Could you do this in-place?

本地使得二维矩阵，旋转90角度。

通过实际数据分析，通过两个步骤的元素交换可实现目标：

1. 按照副主对角线，将对称元素交换
2. 按照行，将对称列元素全部交换

即可达到，使得二维矩阵，本地旋转90个角度。

注意坐标转换规则

public void rotate(int[][] matrix) {
		if (matrix == null)
			return;
		int n = matrix.length;
		// 沿着副对角线旋转
		for (int i = 0; i < n; i++)
			for (int j = 0; j < n - i; j++) {
				int temp = matrix[i][j];
				matrix[i][j] = matrix[n - 1 - j][n - 1 - i];
				matrix[n - 1 - j][n - 1 - i] = temp;
			}
		// 沿着中间水平线旋转
		for (int i = 0; i < n / 2; i++)
			for (int j = 0; j < n; j++) {
				int temp = matrix[i][j];
				matrix[i][j] = matrix[n - 1 - i][j];
				matrix[n - 1 - i][j] = temp;
			}
	}

纯模拟，从外到内一圈一圈的转，但这个方法太慢。

A[i][j] -> A[j][n-1-i] -> A[n-1-i][n-1-j] -> A[n-1-j][i] -> A[i][j]

public void rotate(int[][] matrix) {
		int n = matrix.length;
		for (int layer = 0; layer < n / 2; layer++) {
			int start = layer;
			int end = n - 1 - start;
			for (int i = start; i < end; i++) {
				int offset = i - start;
				int temp = matrix[start][i];
				// left to top;
				matrix[start][i] = matrix[end - offset][start];
				// bottom to left;
				matrix[end - offset][start] = matrix[end][end - offset];
				// right to bottm;
				matrix[end][end - offset] = matrix[i][end];
				// top to right;
				matrix[i][end] = temp;
			}
		}
	}

参考：
https://yq.aliyun.com/articles/3878

https://segmentfault.com/a/1190000007429004