readme.md

48. Rotate Image

Solution

• 有两种方法，第一种以anti-clockwise的方向matrix[i][j] = matrix[N - j][i]; 交换三次，元素就都在正确位置了，但是这种方法如果答OA很容易出错

• 第二种方法现按照reverse diagonal方向swap元素，再以middle row为轴做swap

• follow up: reverse in anti-clock direction by 90 degree: 1) swap around digonal; 2)swap around the middle row

  

  class Solution {
  		//time complexity O(N^2) || space complexity O(1)
      public void rotate(int[][] matrix) {
          int N = matrix.length - 1;
          //swap around the reverse diagonal
          for(int i = 0; i < N; i++){
              for(int j = 0; j < N - i; j++)
                  swap(matrix, i, j, N - j, N - i);
          }
          //swap around the middle row
          for(int i = 0; i < (N + 1) / 2; i++)
              for(int j = 0; j <= N; j++)
                  swap(matrix, i, j, N - i, j);
      }
  
      private void swap(int[][] matrix, int i, int j, int x, int y) {
          int tempt = matrix[i][j];
          matrix[i][j] = matrix[x][y];
          matrix[x][y] = tempt;
      }
  }

  class Solution {
    //time complexity O(N^2) || space O(1)
    public void rotate(int[][] matrix) {
      int N = matrix.length - 1;
      int start = 0, bound = N - 1, y = 0;
      while(start <= bound){
        for(int x = start; x <= bound; x++){
          //anti clockwise swap (x,y) with (N - y, x) three times
          int count = 3;
          int i = x, j = y;
          while(--count >= 0){
            int tempt = matrix[i][j];
            matrix[i][j] = matrix[N - j][i];
            matrix[N - j][i] = tempt;
            int temptI = i;
            i = N - j;
            j = temptI;
          }
        }
        start ++;
        bound --;
        y++;
      }
    }
  }

  class Solution{
     public void rotate(int[][] matrix){
       // swap around the diagonal
       int N = matrix.length;
       for(int i = 0; i < N; i++){
         for(int j = 0; j < i; j++){
           swap(matrix, i, j, j, i);
         }
       }
  
       // swap around the middle row
       for(int i = 0; i < N / 2; i++){
         for(int j = 0; j < N; j++){
           swap(matrix, i, j, N - 1 - i, j);
         }
       }
  
     }
  
     private void swap(int[][] matrix, int i, int j, int x, int y){
       int tempt = matrix[i][j];
       matrix[i][j] = matrix[x][y];
       matrix[x][y] = tempt;
     }
  }