leetcode_gym/_problems/rotateMatrix.md

layout, title, last_solved, category, leetcode_url, status

layout	title	last_solved	category	leetcode_url	status
page	Rotate Matrix	2020-06-10	Matrix	https://leetcode.com/problems/rotate-image/	Solved

Problem

Given input matrix = 
[
  [1,2,3],
  [4,5,6],
  [7,8,9]
],

rotate the input matrix in-place such that it becomes:
[
  [7,4,1],
  [8,5,2],
  [9,6,3]
]

Solution

Transpose and Flip. This is the second time that I'm seeing this and I really worked hard to understand it the first time.

The second time, I was able to:

• recall "the solution"
• code up the solution and get the array indexes and loop ranges correct (things I usually mess up on)

{% highlight python %}

def rotateMatrix(mat):

N = len(mat)

# transpose
for i in range(N):
    for j in range(i, N):

        tmp = mat[i][j]
        mat[i][j] = mat[j][i]
        mat[j][i] = tmp

print( 'after transpose: ',  mat )

# flip horizontally
for i in range(N):
    for j in range(N//2):
        
        # swap j and N-j
        tmp = mat[i][j]
        mat[i][j] = mat[i][N-j-1]
        mat[i][N-j-1] = tmp

print( 'after flip: ',  mat )

{% endhighlight %}