867. Transpose Matrix

867. Transpose Matrix

题目：

Given a matrix A, return the transpose of A.
The transpose of a matrix is the matrix flipped over it’s main diagonal, switching the row and column indices of the matrix.
Example 1:
Input: [[1,2,3],[4,5,6],[7,8,9]]
Output: [[1,4,7],[2,5,8],[3,6,9]]
Example 2:
Input: [[1,2,3],[4,5,6]]
Output: [[1,4],[2,5],[3,6]]
Note:
1.1 <= A.length <= 1000
2.1 <= A[0].length <= 1000

解法一：

O(nm):暴力解，20ms。

class Solution {
public:
    vector<vector<int>> transpose(vector<vector<int>>& A) {
        vector<vector<int>> res;
        if(A.size() == 0)
            return res;
        for(int j = 0;j < A[0].size();j++)
        {
            vector<int> temp;
            for(int i = 0;i < A.size();i++)
            {
                temp.push_back(A[i][j]);
            }
            res.push_back(temp);
            temp.clear();
        }
        return res;
    }
};

解法二：

16ms，将vector初始化成二维数组，并且写入数据时按照res的大小来写，先 j 再 i 即每行每行的写，耗时会更少。

class Solution {
public:
    vector<vector<int>> transpose(vector<vector<int>>& A) {
        int n = A.size();
        int m = A[0].size();
        vector<vector<int>> res(m,vector<int>(n));
        for(int j = 0;j < A[0].size();j++)
        {
            for(int i = 0;i < A.size();i++)
            {
                res[j][i] = A[i][j];
            }
        }
        return res;
    }
};