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;    }};