LeetCode ： Pascal's Triangle

常规，一层一层来

vector<vector<int>> generate(int numRows){    vector<vector<int>> res;    if(numRows < 1)        return res;    vector<int> first(1, 1);    res.push_back(first);    for(int i = 1; i < numRows; i++)    {        vector<int> v(1, 1);        for(int j = 1; j < i; j ++)        {            v.push_back(first[j - 1] + first[j]);        }        v.push_back(1);             res.push_back(v);        first = v;    }    return res;}