leetcode 118 Pascal's Triangle

给定一整数，求Pascal三角形。

vector<vector<int>> generate(int numRows) {
    vector<vector<int> > ret;
    
    for (int i = 0; i < numRows; ++i) {
        vector<int> ins;
        for (int j = 0; j <= i; ++j) {
            if (j == 0 || j == i)
                ins.push_back(1);
            else
                ins.push_back(ret[i - 1][j - 1] + ret[i - 1][j]);
        }
        ret.push_back(move(ins));
    }
    return move(ret);
}