algorithm notes: leetcode#566 reshape the matrix

Problem

In MATLAB, there is a very useful function called ‘reshape’, which can reshape a matrix into a new one with different size but keep its original data.

You’re given a matrix represented by a two-dimensional array, and two positive integers r and c representing the row number and column number of the wanted reshaped matrix, respectively.

The reshaped matrix need to be filled with all the elements of the original matrix in the same row-traversing order as they were.

If the ‘reshape’ operation with given parameters is possible and legal, output the new reshaped matrix; Otherwise, output the original matrix.

Example 1:

Input:
nums =
[[1,2],
[3,4]]
r = 1, c = 4
Output:
[[1,2,3,4]]
Explanation:
The row-traversing of nums is [1,2,3,4]. The new reshaped matrix is a 1 * 4 matrix, fill it row by row by using the previous list.

Example 2:

Input:
nums =
[[1,2],
[3,4]]
r = 2, c = 4
Output:
[[1,2],
[3,4]]
Explanation:
There is no way to reshape a 2 * 2 matrix to a 2 * 4 matrix. So output the original matrix.

Note:

1. The height and width of the given matrix is in range [1, 100].
2. The given r and c are all positive.

Solution

Analysis

Iterate each element and flatten the matrix. Then change it to the expected shape.

Python implementation

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class :
    def matrixReshape(self, nums, r, c):
        """
        :type nums: List[List[int]]
        :type r: int
        :type c: int
        :rtype: List[List[int]]
        """
        if r * c != len(nums) * len(nums[0]):
            return nums
        flatten = sum(nums, [])
        return [flatten[i*c:(i+1)*c] for i in range(r)]

Or,

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class (object):
    def matrixReshape(self, nums, r, c):
        """
        :type nums: List[List[int]]
        :type r: int
        :type c: int
        :rtype: List[List[int]]
        """
        flatten = []
        for row in nums:
            flatten += row
        return [flatten[i*c : i*c+c] for i in range(r)] if r * c == len(flatten) else nums

Java implementation

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class  {
    public int[][] matrixReshape(int[][] nums, int r, int c) {
        if(r * c != nums.length * nums[0].length){ return nums; }
        int[] flatten = new int[r * c];
        for(int i = 0; i < nums.length; i++){
            for(int j = 0; j < nums[0].length; j++){
                flatten[i*nums[0].length + j] = nums[i][j];
            }
        }
        int[][] ans = new int[r][];
        for(int i = 0; i < r; i++){
            ans[i] = Arrays.copyOfRange(flatten, i*c, i*c+c);
        }
        return ans;
    }
}

Time complexity

O(MN).

Space complexity

O(MN).

Link

566. Reshape the Matrix
566. 重塑矩阵
(中文版) 算法笔记: 力扣#566 重塑矩阵