 ## Code challenge: Rotate matrix in-place in 90 degrees

May 10, 2022
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Sometimes I do some coding challenges to challenge myself.

This time I had a challenge to rotate a matrix in 90 degrees in-place. I had a good first idea to solve this but I struggled a little bit with the details. To help my future me and maybe some other people I share my considerations. ``````var matrix = new int[,] {{10, 11, 12, 13}, {14, 15, 16, 17}, {18, 19, 20, 21}, {22, 23, 24, 25}};
printMatrix(matrix);
rotate(matrix);

static void rotate(int[,] matrix)
{
var l = 0;
var r = matrix.GetLength(0) - 1;
var t = 0;
var b = matrix.GetLength(1) - 1;

while (l < r)
{
// run through the "slice"
for (var i = l; i <= r - 1; i++)
{
// cache
var tl = matrix[t, l + i];

// exchange in circle
matrix[t, l + i] = matrix[b - i, l]; // left -> top
matrix[b - i, l] = matrix[b, r - i]; // bottom -> left
matrix[b, r - i] = matrix[t + i, r]; // right -> bottom
matrix[t + i, r] = tl; // top -> right
}

l++;
r--;
t++;
b--;
}
}``````
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