logseq_notes/journals/2023_04_21.md

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• Leetcode - Rotate Image id:: 64422d1f-5db3-435d-ab4e-2b9fe5fd47cd collapsed:: true
  • Times:
    • Time when completed: 14:28
    • Time taken to complete: 15 mins ( with solution in mind )
    • DONE Revisions: SCHEDULED: <2023-05-26 Fri> :LOGBOOK:
      • State "DONE" from "LATER" [2023-04-28 Fri 16:45]

      • State "DONE" from "LATER" [2023-05-23 Tue 20:26] :END:

  • Tags:
    • Algorithms: #math
    • Data structures: #array
    • Difficulty: #difficulty_easy
    • Platforms: #leetcode
  • Links:
    • link to the problem
  • Problem:
    • 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.
  • Examples:
    • https://assets.leetcode.com/uploads/2020/08/28/mat1.jpg
    • https://assets.leetcode.com/uploads/2020/08/28/mat2.jpg

    • 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
  • Thoughts:
    • Intuition:
      • The solution is tricky, which requires you to use math.
      • In order to do this, first, transpose, then reflect the matrix.
  • Solution:
    • java

      • class Solution {
          private void swap(int[][] matrix, int a, int b, int x, int y) {
            int temp = matrix[a][b];
            matrix[a][b] = matrix[x][y];
            matrix[x][y] = temp;
          }
          public void rotate(int[][] matrix) {
            // transpose and reflect
            for (int i = 0; i < matrix.length; i++) {
              for (int j = 0; j < i; j++) {
                swap(matrix, i, j, j, i);
              }
            }
        
            for (int i = 0; i < matrix.length; i++) {
              for (int j = 0; j < matrix.length / 2; j++) {
                swap(matrix, i, j, i, matrix.length - j - 1);
              }
            }
          }
        }
        

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