Question

48. Rotate Image

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Description

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.

 

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

Solutions

Solution 1: In-place Rotation

According to the problem requirements, we actually need to rotate $matrix[i][j]$ to $matrix[j][n - i - 1]$.

We can first flip the matrix upside down, that is, swap $matrix[i][j]$ and $matrix[n - i - 1][j]$, and then flip the matrix along the main diagonal, that is, swap $matrix[i][j]$ and $matrix[j][i]$. This way, we can rotate $matrix[i][j]$ to $matrix[j][n - i - 1]$.

The time complexity is $O(n^2)$, where $n$ is the side length of the matrix. The space complexity is $O(1)$.

class Solution:
  def rotate(self, matrix: List[List[int]]) -> None:
    n = len(matrix)
    for i in range(n >> 1):
      for j in range(n):
        matrix[i][j], matrix[n - i - 1][j] = matrix[n - i - 1][j], matrix[i][j]
    for i in range(n):
      for j in range(i):
        matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]

class Solution {
  public void rotate(int[][] matrix) {
    int n = matrix.length;
    for (int i = 0; i < n >> 1; ++i) {
      for (int j = 0; j < n; ++j) {
        int t = matrix[i][j];
        matrix[i][j] = matrix[n - i - 1][j];
        matrix[n - i - 1][j] = t;
      }
    }
    for (int i = 0; i < n; ++i) {
      for (int j = 0; j < i; ++j) {
        int t = matrix[i][j];
        matrix[i][j] = matrix[j][i];
        matrix[j][i] = t;
      }
    }
  }
}

class Solution {
public:
  void rotate(vector<vector<int>>& matrix) {
    int n = matrix.size();
    for (int i = 0; i < n >> 1; ++i) {
      for (int j = 0; j < n; ++j) {
        swap(matrix[i][j], matrix[n - i - 1][j]);
      }
    }
    for (int i = 0; i < n; ++i) {
      for (int j = 0; j < i; ++j) {
        swap(matrix[i][j], matrix[j][i]);
      }
    }
  }
};

func rotate(matrix [][]int) {
  n := len(matrix)
  for i := 0; i < n>>1; i++ {
    for j := 0; j < n; j++ {
      matrix[i][j], matrix[n-i-1][j] = matrix[n-i-1][j], matrix[i][j]
    }
  }
  for i := 0; i < n; i++ {
    for j := 0; j < i; j++ {
      matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
    }
  }
}

/**
 Do not return anything, modify matrix in-place instead.
 */
function rotate(matrix: number[][]): void {
  matrix.reverse();
  for (let i = 0; i < matrix.length; ++i) {
    for (let j = 0; j < i; ++j) {
      const t = matrix[i][j];
      matrix[i][j] = matrix[j][i];
      matrix[j][i] = t;
    }
  }
}

impl Solution {
  pub fn rotate(matrix: &mut Vec<Vec<i32>>) {
    let n = matrix.len();
    for i in 0..n / 2 {
      for j in 0..n {
        let t = matrix[i][j];
        matrix[i][j] = matrix[n - i - 1][j];
        matrix[n - i - 1][j] = t;
      }
    }
    for i in 0..n {
      for j in 0..i {
        let t = matrix[i][j];
        matrix[i][j] = matrix[j][i];
        matrix[j][i] = t;
      }
    }
  }
}

/**
 * @param {number[][]} matrix
 * @return {void} Do not return anything, modify matrix in-place instead.
 */
var rotate = function (matrix) {
  matrix.reverse();
  for (let i = 0; i < matrix.length; ++i) {
    for (let j = 0; j < i; ++j) {
      [matrix[i][j], matrix[j][i]] = [matrix[j][i], matrix[i][j]];
    }
  }
};

public class Solution {
  public void Rotate(int[][] matrix) {
    int n = matrix.Length;
    for (int i = 0; i < n >> 1; ++i) {
      for (int j = 0; j < n; ++j) {
        int t = matrix[i][j];
        matrix[i][j] = matrix[n - i - 1][j];
        matrix[n - i - 1][j] = t;
      }
    }
    for (int i = 0; i < n; ++i) {
      for (int j = 0; j < i; ++j) {
        int t = matrix[i][j];
        matrix[i][j] = matrix[j][i];
        matrix[j][i] = t;
      }
    }
  }
}