867. Transpose Matrix

题目描述：

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

解题思路：

暴力遍历原来矩阵的每个元素，放到输出矩阵的对应位置。

由于veector的内存分配机制，由于已知输出矩阵的大小，所以在每个vector定义时指定空间大小会节省运行时间。

代码：

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