Leetcode - Rotate Image

最新推荐文章于 2020-11-24 22:28:40 发布

原创最新推荐文章于 2020-11-24 22:28:40 发布 · 142 阅读

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本文介绍了一种矩阵旋转算法，通过示例代码展示了如何实现矩阵的顺时针和逆时针旋转。提供了两种高效的实现方法：一是先上下颠倒再进行转置；二是先转置再左右颠倒。同时给出了具体的Java代码实现。

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[分析]
自己的思路：从外到内一圈圈顺时针旋转90度，坐标映射问题。
Leetcode讨论区有很多有趣巧妙的思路，列举两个点赞率较高思路：
1）上下颠倒，然后转置
/*
* clockwise rotate
* first reverse up to down, then swap the symmetry
* 1 2 3 7 8 9 7 4 1
* 4 5 6 => 4 5 6 => 8 5 2
* 7 8 9 1 2 3 9 6 3
*/
2）转置，然后左右颠倒
若需要逆时针，则分别是：
1）左右颠倒，然后转置
2）转置，然后上下颠倒
[ref]
[url]https://leetcode.com/discuss/20589/a-common-method-to-rotate-the-image[/url]


public class Solution {
    public void rotate1(int[][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0)
            return;
        int rowBeg = 0, rowEnd = matrix.length - 1;
        int colBeg = 0, colEnd = matrix[0].length - 1;
        while (rowBeg <= rowEnd) {
            int offset = colEnd - colBeg - 1;
            for (int j = 0; j <= offset; j++) {
                int save = matrix[rowBeg][colBeg + j];
                matrix[rowBeg][colBeg + j] = matrix[rowEnd - j][colBeg];
                matrix[rowEnd - j][colBeg] = matrix[rowEnd][colEnd - j];
                matrix[rowEnd][colEnd - j] = matrix[rowBeg + j][colEnd];
                matrix[rowBeg + j][colEnd] = save;
            }
            rowBeg++; rowEnd--;
            colBeg++; colEnd--;
        }
    }
    // first transpose, then flip the matrix horizontally
    public void rotate(int[][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0)
            return;
        int rows = matrix.length, cols = matrix[0].length;
        for (int i = 0; i < rows; i++) {
            for (int j = i + 1; j < cols; j++) {
                int tmp = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = tmp;
            }
        }
        int half = cols / 2;
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < half; j++) {
                int tmp = matrix[i][j];
                matrix[i][j] = matrix[i][cols -1 - j];
                matrix[i][cols -1 - j] = tmp;
            }
        }
    }
}