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  • 48. Rotate Image ~

    题目:

    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?

    链接:  http://leetcode.com/problems/rotate-image/

    4/15/2017

    4ms, 8%

     1 public class Solution {
     2     public void rotate(int[][] matrix) {
     3         int length = matrix.length;
     4         for (int i = 0; i < (length + 1) / 2; i++) {
     5             for (int j = i; j < length - 1 - i; j++) {
     6                 int tmp = matrix[i][j];
     7                 matrix[i][j] = matrix[length - 1 - j][i];
     8                 matrix[length - 1 - j][i] = matrix[length - 1 - i][length - 1 - j];
     9                 matrix[length - 1 - i][length - 1 - j] = matrix[j][length - 1 - i];
    10                 matrix[j][length - 1 - i] = tmp;
    11             }
    12         }
    13         return;
    14     }
    15 }

    别人的答案,聪明人啊,先上下对称,再左右对称

    https://discuss.leetcode.com/topic/6796/a-common-method-to-rotate-the-image

     1 /*
     2  * clockwise rotate
     3  * first reverse up to down, then swap the symmetry 
     4  * 1 2 3     7 8 9     7 4 1
     5  * 4 5 6  => 4 5 6  => 8 5 2
     6  * 7 8 9     1 2 3     9 6 3
     7 */
     8 void rotate(vector<vector<int> > &matrix) {
     9     reverse(matrix.begin(), matrix.end());
    10     for (int i = 0; i < matrix.size(); ++i) {
    11         for (int j = i + 1; j < matrix[i].size(); ++j)
    12             swap(matrix[i][j], matrix[j][i]);
    13     }
    14 }

    类似的,先中心对称,再左右对称

    https://discuss.leetcode.com/topic/9744/ac-java-in-place-solution-with-explanation-easy-to-understand

     1 public class Solution {
     2     public void rotate(int[][] matrix) {
     3         for(int i = 0; i<matrix.length; i++){
     4             for(int j = i; j<matrix[0].length; j++){
     5                 int temp = 0;
     6                 temp = matrix[i][j];
     7                 matrix[i][j] = matrix[j][i];
     8                 matrix[j][i] = temp;
     9             }
    10         }
    11         for(int i =0 ; i<matrix.length; i++){
    12             for(int j = 0; j<matrix.length/2; j++){
    13                 int temp = 0;
    14                 temp = matrix[i][j];
    15                 matrix[i][j] = matrix[i][matrix.length-1-j];
    16                 matrix[i][matrix.length-1-j] = temp;
    17             }
    18         }
    19     }
    20 }

    这位大哥对python太了解了,还是写下来,有些最基本的我看来也不了解

    https://discuss.leetcode.com/topic/15295/seven-short-solutions-1-to-7-lines

    A[::-1]是把Python上下反转,这个没问题,zip() in conjunction with the * operator can be used to unzip a list.

    举例说,是否inplace不讨论,不过最后是list of tuple,也有点怀疑啊

    A = [[1,2,3,4], [5,6,7,8], [9,10,11,12], [13,14,15,16]]
    A[::-1] = [[13, 14, 15, 16], [9, 10, 11, 12], [5, 6, 7, 8], [1, 2, 3, 4]]
    zip(*A[::-1])等于是把4个list进行unzip,得
    [(13, 9, 5, 1), (14, 10, 6, 2), (15, 11, 7, 3), (16, 12, 8, 4)]
    class Solution:
        def rotate(self, A):
            A[:] = zip(*A[::-1])

    发现这位大哥也认识到了这点,这个算法解决了这个问题, map等于将A当中每个tuple改成了list

    map(functioniterable...)

    class Solution:
        def rotate(self, A):
            A[:] = map(list, zip(*A[::-1]))

    更多讨论:

    https://discuss.leetcode.com/category/56/rotate-image

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  • 原文地址:https://www.cnblogs.com/panini/p/6718064.html
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