Step 4 of 6

2D vectors

A vector of vectors makes a grid whose size is decided at runtime:

std::vector<std::vector<int>> grid(rows, std::vector<int>(cols, 0));
grid[r][c] = 5;

Your turn: write std::vector<std::vector<int>> transpose(const std::vector<std::vector<int>>& m) that swaps rows and columns. A 2x3 matrix becomes 3x2. You can assume every row has the same length and there's at least one row.

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