Today I have done leetcode's August LeetCoding Challenge with cpp.

August LeetCoding Challenge 30

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:

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]]

Constraints:

• n == matrix.length == matrix[i].length
• 1 <= n <= 20
• -1000 <= matrix[i][j] <= 1000

Solution

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] -> ...

// Accepted
// 21/21 cases passed (6 ms)
// Your runtime beats 23.35 % of cpp submissions
// Your memory usage beats 33.5 % of cpp submissions (7.1 MB)