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<int> v;        vector<vector<int>> vv;        if(numRows == 0)            return vv;        v.push_back(1);        vv.push_back(v);        if(numRows == 1)            return vv;        v.push_back(1);        vv.push_back(v);        if(numRows == 2)            return vv;        for(int i=2; i<numRows; i++)        {            v.clear();            v.push_back(1);            for(int j=1; j<i; j++)                v.push_back(vv[i-1][j-1] + vv[i-1][j]);            v.push_back(1);            vv.push_back(v);        }        return vv;    }};