leetcode 566. Reshape the Matrix (矩阵的reshape）-优快云博客

本文链接：https://blog.youkuaiyun.com/level_code/article/details/109588197

介绍了一个名为'matrixReshape'的函数，该函数能够将一个二维数组表示的矩阵重塑为指定行数和列数的新矩阵，同时保持元素的行遍历顺序不变。若重塑操作合法，则输出重塑后的矩阵；反之，则返回原始矩阵。

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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.

写一个reshape函数，使原矩阵改为r * c的矩阵，按行遍历。如果无法满足改成r * c的条件，就返回原数组。

思路：
主要是原矩阵的index和重构后r * c的index转换问题
假设原矩阵是m行n列，重构后是r行c列
可以通过转换到一维的中间index
重构后r行c列的index转换到一维：k = c*i + j (i行j列）
转换到m行n列index为 k/n行, k%n列 (可以参照如何由k转换到i, j)

    public int[][] matrixReshape(int[][] nums, int r, int c) {
        if(nums == null || nums.length == 0) {
            return nums;
        }
        
        int m = nums.length;
        int n = nums[0].length;
        
        if(m*n != r*c) {
            return nums;
        }
        
        int[][] result = new int[r][c];
        for(int i = 0; i < r; i++) {
            for(int j = 0; j < c; j ++) {
                int k = c*i +j;
                result[i][j] = nums[k/n][k%n];
            }
        }
        return result;
    }