LeetCode-Pascal's Triangle

class Solution {
public:
vector<vector<int> > generate(int numRows) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
vector<vector<int> > ans;
vector<int> pre;
for (int i = 0; i < numRows; ++i)
{
vector<int> cur(i + 1);
cur[0] = cur[i] = 1;
for (int j = 1; j < i; ++j)
cur[j] = pre[j - 1] + pre[j];
ans.push_back(cur);
pre = cur;
}
return ans;
}
};