118Pascal's Triangle

class Solution {public:    vector<vector<int> > generate(int numRows) {        vector<int>vec;        vector<vector <int> >ans;        if(numRows==0)            return ans;        vec.push_back(1);        ans.push_back(vec);        for(int i=1; i<=numRows-1; i++){            int p=0, a=0, b=1;            for(;p<i;p++){                vec[p]=a+b;                a=b;                if(p+1<i)                    b=vec[p+1];            }            vec.push_back(1);            ans.push_back(vec);        }        return ans;    }};