2021-04-25 Daily-Challenge

Posted on:
Posted modified:
In: DailyChallenge

Today is Sunday, I gonna review the tasks I've done this week, and finish today's leetcode's April LeetCoding Challenge with cpp.

April LeetCoding Challenge 25

Description

Rotate Image

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:

img

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

Example 2:

img

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

Solution

done at spring festival

class Solution {
public:
    void rotate(vector<vector<int>>& matrix) {
        int n = matrix.size();
        int low = n / 2;
        int high = n - low;
        for(int j = 0; j < high; ++j) {
            for(int i = 0; i+j < n-1-j; ++i) {
                swap(matrix[j][j+i], matrix[j+i][n-1-j]);
                swap(matrix[j][j+i], matrix[n-1-i-j][j]);
                swap(matrix[n-1-j][n-1-i-j], matrix[n-1-i-j][j]);
            }
        }
    }
};

// [0, 0], [0, n-1], [n-1, n-1], [n-1, 0] -> [n-1, 0], [0, 0], [0, n-1], [n-1, n-1]
// [0, 1], [1, n-1], [n-1, n-2], [n-2, 0] -> [n-2, 0], [0, 1], [1, n-1], [n-1, n-2]
// ...
// [0, i], [i, n-1], [n-1, n-1-i], [n-1-i, 0] -> ...
// ...
// [0, n-2], [n-2, n-1], [n-1, 1], [1, 0] -> ...

// [1, 1], [1, n-2], [n-2, n-2], [n-2, 1] -> ...
// [1, 2], [2, n-2], [n-2, n-3], [n-3, 1] -> ...
// ...
// [1, 1+i], [1+i, n-2], [n-2, n-2-i], [n-2-i, 1] -> ...

// [j, j+i], [j+i, n-1-j], [n-1-j, n-1-i-j], [n-1-i-j, j] -> ...
Algorithm