Rotate a matrix by 90 degree without using any extra space | Set 2

Given a square matrix, turn it by 90 degrees in anti-clockwise direction without using any extra space.

Examples:

Input:
 1  2  3
 4  5  6
 7  8  9
Output:
 3  6  9 
 2  5  8 
 1  4  7 
Rotated the input matrix by
90 degrees in anti-clockwise direction.

Input:
 1  2  3  4 
 5  6  7  8 
 9 10 11 12 
13 14 15 16 
Output:
 4  8 12 16 
 3  7 11 15 
 2  6 10 14 
 1  5  9 13
Rotated the input matrix by
90 degrees in anti-clockwise direction.

An approach that requires extra space is already discussed in a different article:
Inplace rotate square matrix by 90 degrees | Set 1

This post discusses the same problem with a different approach which is space-optimized.

Approach: The idea is to find the transpose of the matrix and then reverse the columns of the transposed matrix.
Here is an example to show how this works.



Algorithm:

  1. To solve the given problem there are two tasks. 1st is finding the transpose and second is reversing the columns without using extra space
  2. A transpose of a matrix is when the matrix is flipped over its diagonal, i.e the row index of an element becomes the column index and vice versa. So to find the transpose interchange the elements at position (i, j) with (j, i). Run two loops, the outer loop from 0 to row count and inner loop from 0 to index of the outer loop.
  3. To reverse the column of the transposed matrix, run two nested loops, the outer loop from 0 to column count and inner loop from 0 to row count/2, interchange elements at (i, j) with (i, row[count-1-j]), where i and j are indices of inner and outer loop respectively.
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// C++ program for left
// rotation of matrix by 90 degree
// without using extra space
#include <bits/stdc++.h>
using namespace std;
#define R 4
#define C 4
  
// After transpose we swap
// elements of column
// one by one for finding left
// rotation of matrix
// by 90 degree
void reverseColumns(int arr[R][C])
{
    for (int i = 0; i < C; i++)
        for (int j = 0, k = C - 1;
             j < k; j++, k--)
            swap(
                arr[j][i], arr[k][i]);
}
  
// Function for do transpose of matrix
void transpose(int arr[R][C])
{
    for (int i = 0; i < R; i++)
        for (int j = i; j < C; j++)
            swap(arr[i][j], arr[j][i]);
}
  
// Function for print matrix
void printMatrix(int arr[R][C])
{
    for (int i = 0; i < R; i++) {
        for (int j = 0; j < C; j++)
            cout << arr[i][j] << " ";
        cout << '\n';
    }
}
  
// Function to anticlockwise
// rotate matrix by 90 degree
void rotate90(int arr[R][C])
{
    transpose(arr);
    reverseColumns(arr);
}
  
// Driven code
int main()
{
    int arr[R][C] = { { 1, 2, 3, 4 },
                      { 5, 6, 7, 8 },
                      { 9, 10, 11, 12 },
                      { 13, 14, 15, 16 } };
    rotate90(arr);
    printMatrix(arr);
    return 0;
}
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// JAVA Code for left Rotation of a
// matrix by 90 degree without using
// any extra space
import java.util.*;
  
class GFG {
  
    // After transpose we swap elements of
    // column one by one for finding left
    // rotation of matrix by 90 degree
    static void reverseColumns(int arr[][])
    {
        for (int i = 0; i < arr[0].length; i++)
            for (int j = 0, k = arr[0].length - 1;
                 j < k; j++, k--) {
                int temp = arr[j][i];
                arr[j][i] = arr[k][i];
                arr[k][i] = temp;
            }
    }
  
    // Function for do transpose of matrix
    static void transpose(int arr[][])
    {
        for (int i = 0; i < arr.length; i++)
            for (int j = i; j < arr[0].length;
                 j++) {
                int temp = arr[j][i];
                arr[j][i] = arr[i][j];
                arr[i][j] = temp;
            }
    }
  
    // Function for print matrix
    static void printMatrix(int arr[][])
    {
        for (int i = 0; i < arr.length; i++) {
            for (int j = 0; j < arr[0].length;
                 j++)
                System.out.print(arr[i][j] + " ");
            System.out.println("");
        }
    }
  
    // Function to anticlockwise rotate
    // matrix by 90 degree
    static void rotate90(int arr[][])
    {
        transpose(arr);
        reverseColumns(arr);
    }
  
    /* Driver program to test above function */
    public static void main(String[] args)
    {
        int arr[][] = { { 1, 2, 3, 4 },
                        { 5, 6, 7, 8 },
                        { 9, 10, 11, 12 },
                        { 13, 14, 15, 16 } };
  
        rotate90(arr);
        printMatrix(arr);
    }
}
  
// This code is contributed by Arnav Kr. Mandal.
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// C# program for left rotation
// of matrix by 90 degree
// without using extra space
using System;
  
class GFG {
    static int R = 4;
    static int C = 4;
  
    // After transpose we swap
    // elements of column one
    // by one for finding left
    // rotation of matrix by
    // 90 degree
    static void reverseColumns(int[, ] arr)
    {
        for (int i = 0; i < C; i++)
            for (int j = 0, k = C - 1;
                 j < k; j++, k--) {
                int temp = arr[j, i];
                arr[j, i] = arr[k, i];
                arr[k, i] = temp;
            }
    }
  
    // Function for do
    // transpose of matrix
    static void transpose(int[, ] arr)
    {
        for (int i = 0; i < R; i++)
            for (int j = i; j < C; j++) {
                int temp = arr[j, i];
                arr[j, i] = arr[i, j];
                arr[i, j] = temp;
            }
    }
  
    // Function for print matrix
    static void printMatrix(int[, ] arr)
    {
  
        for (int i = 0; i < R; i++) {
            for (int j = 0; j < C; j++)
                Console.Write(arr[i, j] + " ");
            Console.WriteLine("");
        }
    }
  
    // Function to anticlockwise
    // rotate matrix by 90 degree
    static void rotate90(int[, ] arr)
    {
        transpose(arr);
        reverseColumns(arr);
    }
  
    // Driver code
    static void Main()
    {
        int[, ] arr = { { 1, 2, 3, 4 },
                        { 5, 6, 7, 8 },
                        { 9, 10, 11, 12 },
                        { 13, 14, 15, 16 } };
  
        rotate90(arr);
        printMatrix(arr);
    }
  
    // This code is contributed
    // by Sam007
}
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# Python 3 program for left rotation of matrix by 90
# degree without using extra space
  
R = 4
C = 4
   
# After transpose we swap elements of column
# one by one for finding left rotation of matrix
# by 90 degree
def reverseColumns(arr):
    for i in range(C):
        j = 0
        k = C-1
        while j < k:
            t = arr[j][i]
            arr[j][i] = arr[k][i]
            arr[k][i] = t
            j += 1
            k -= 1
    
# Function for do transpose of matrix
def transpose(arr):
    for i in range(R):
        for j in range(i, C):
            t = arr[i][j]
            arr[i][j] = arr[j][i]
            arr[j][i] = t
   
# Function for print matrix
def printMatrix(arr):
    for i in range(R):
        for j in range(C):
            print(str(arr[i][j]), end =" ")
        print()
   
# Function to anticlockwise rotate matrix
# by 90 degree
def rotate90(arr):
    transpose(arr)
    reverseColumns(arr)
   
# Driven code
arr = [[1, 2, 3, 4],
        [5, 6, 7, 8],
        [9, 10, 11, 12],
        [13, 14, 15, 16]
    ];
rotate90(arr)
printMatrix(arr)
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<?php
// PHP program for left rotation of matrix by 90
  
$R = 4;
$C = 4;
// Function to rotate the matrix by 90 degree
function reverseColumns(&$arr)
{
    global $C;
    for ($i = 0; $i < $C; $i++)
    {
        for ($j = 0, $k = $C - 1; $j < $k; $j++, $k--)
        {
            $t = $arr[$j][$i];
            $arr[$j][$i] = $arr[$k][$i];
            $arr[$k][$i] = $t;
        }
    }       
}
   
// Function for transpose of matrix
function transpose(&$arr)
{
    global $R, $C;
    for ($i = 0; $i < $R; $i++)
    {
        for ($j = $i; $j < $C; $j++)
        {
            $t = $arr[$i][$j];
            $arr[$i][$j] = $arr[$j][$i];
            $arr[$j][$i] = $t;
        }
    }
}
   
// Function for display the matrix
function printMatrix(&$arr)
{
    global $R, $C;
    for ($i = 0; $i < $R; $i++) {
        for ($j = 0; $j < $C; $j++)
            echo $arr[$i][$j]." ";
        echo "\n";
    }
}
   
// Function to anticlockwise rotate matrix
// by 90 degree
function rotate90(&$arr)
{
    transpose($arr);
    reverseColumns($arr);
}
   
// Driven code
  
$arr = array( array( 1, 2, 3, 4 ),
                 array( 5, 6, 7, 8 ),
                 array( 9, 10, 11, 12 ),
                 array( 13, 14, 15, 16 ) );
rotate90($arr);
printMatrix($arr);
return 0;
?>
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Output:
4 8 12 16 
3 7 11 15 
2 6 10 14 
1 5 9 13

Complexity Analysis:

Implementation: Let’s see a method of Python numpy that can be used to arrive at the particular solution.

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# Alternative implementation using numpy
import numpy
   
# Driven code 
arr = [[1, 2, 3, 4], 
        [5, 6, 7, 8], 
        [9, 10, 11, 12], 
        [13, 14, 15, 16
    ]; 
   
# Define flip algorith (Note numpy.flip is a builtin f
# unction for versions > v.1.12.0)
def flip(m, axis):
    if not hasattr(m, 'ndim'):
        m = asarray(m)
    indexer = [slice(None)] * m.ndim
    try:
        indexer[axis] = slice(None, None, -1)
    except IndexError:
        raise ValueError("axis =% i is invalid for the % i-dimensional input array"
                         % (axis, m.ndim))
    return m[tuple(indexer)]
   
# Transpose the matrix
trans = numpy.transpose(arr)
   
# Flip the matrix anti-clockwise (1 for clockwise)
flipmat = flip(trans, 0)
   
print("\nnumpy implementation\n")
print(flipmat)
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Output:
numpy implementation

[[ 4  8 12 16]
 [ 3  7 11 15]
 [ 2  6 10 14]
 [ 1  5  9 13]]

Note: The above steps/programs do left (or anticlockwise) rotation. Let’s see how to do the right rotation or clockwise rotation. The approach would be similar. Find the transpose of the matrix and then reverse the rows of the transposed matrix.
This is how it is done.

This article is contributed by DANISH_RAZA . If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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