class Solution {public:    vector<vector<int> > generate(int numRows) {        vector<vector<int> > res;        if (numRows < 1) return res;                vector<int> row(1, 1);        res.push_back(row);                for (int i=2; i<=numRows; i++) {            // vector<int> &last = res.back();            res.push_back(vector<int>(i, 1));            vector<int> &crow = res.back();                        int half = i / 2;            int j = 1;            for (; j<half; j++) {                crow[i-j-1] = crow[j] = res[i-2][j] + res[i-2][j-1];            }            if (i & 0x1) {                crow[j] = res[i-2][j] * 2;            }        }        return res;    }};