LeetCode : Pascal's Triangle

Given numRows, generate the first numRows of Pascal’s triangle.

For example, given numRows = 5,
Return

[
[1],
[1,1],
[1,2,1],
[1,3,3,1],
[1,4,6,4,1]
]

class Solution {
public:
    vector<vector<int> > generate(int numRows) {
        vector<vector<int> >vec(numRows);
        for (int i = 0; i < numRows; ++i)
        {
            vec[i].resize(i+1);
        }
        for (int i = 0; i < numRows; ++i)
        {
            vec[i][0] = 1;
            vec[i][i] = 1;
        }
        for (int i = 1; i < numRows; ++i)
        {
            for (int j = 1; j < i; ++j)
            {
                vec[i][j] = vec[i - 1][j-1] + vec[i - 1][j];
            }
        }
        return vec;
    }
};