class Solution {

public:

vector<vector<int>> generate(int numRows) {

vector < vector<int>> a;

for (int i = 0; i < numRows; i++)

{

vector<int>b(i+1,0);

b[0]= b[i] = 1;

int j = 1;

while (i-j>0)

{

b[i - j] = a[i - 1][i-j] + a[i - 1][i -j-1];

j++;

}

a.push_back(b);

}

return a;

}

};