# Rotate Matrix

## Question ([LC.48](https://leetcode.com/problems/rotate-image/))

> Given a mxn matrix, can you rotate it clockwise and counter-clockwise? Can you do it in place?

## Analysis

There are two approaches. One way to do it layer by layer. The second approach requires some linear algebra knowledge. Here we'll discuss the second approach - transposing and flipping the matrix.

## Transpose

Transpose the matrix first n x m result\[x]\[y] = matrix\[y]\[x] m x n

```java
private int[][] transpose(int[][] matrix) {
    int m = matrix.length;
    int n = matrix[0].length;
    int[][] result = new int[n][m];

    for (int y = 0; y < m; y++) {
        for (int x = 0; x < n; x++) {
            result[x][y] = matrix[y][x];
        }
    }

    return result;
}
```

## Vertical/Horizontal Flip

If rotate clockwise, flip horizontally.

If rotate counterclockwise, flip vertically.

```java
private void horizontalFlip(int[][] matrix) {
    int m = matrix.length;
    int n = matrix[0].length;

    for (int y = 0; y < m; y++) {
        for (int x = 0; x < n/2; x++) {
            int temp = matrix[y][x];
            matrix[y][x] = matrix[y][n - 1 - x];
            matrix[y][n - 1- x] = temp;
        }
    }
}

private void verticalFlip(int[][] matrix) {
    int m = matrix.length;
    int n = matrix[0].length;

    for (int x = 0; x < n; x++) {
        for (int y = 0; y < m/2; y++) {
            int temp = matrix[y][x];
            matrix[y][x] = matrix[m - 1 - y][x];
            matrix[m - 1 - y][x] = temp;
        }
    }
}
```

## Rotate Matrix

```java
public int[][] rotateMatrix(int[][] matrix, int flag) {
    if (matrix == null || matrix.length == 0)
        return matrix;
    if (matrix[0] == null || matrix[0].length == 0)
        return matrix;
    int[][] result;
    result = transpose(matrix);

    if (flag == 1) // rotate clockwise
        horizontalFlip(result);
    else // rotate counterclockwise
        verticalFlip(result);
    return result;
}
```

## Note

Rotation in-place has very limited options in terms of operation. You pretty much have to figure out a way that only uses swapping elements and holding a temp variable. Swapping horizontally, vertically, diagonally, multiple times (with different substrings), etc.


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