class Solution {public:    vector<vector<int> > generate(int numRows) {        vector<vector<int> > res;//最终结果        vector<int>  mem;        if(numRows<=0)//参数是0的情况不要忘记           return res;                   mem.push_back(1);        res.push_back(mem);        if(numRows==1)            return res;        mem.push_back(1);        res.push_back(mem);        if(numRows==2)            return res;                for(int i=2;i<numRows;i++){            mem.clear();            mem.push_back(1);            int len  =   res[i-1].size();            for(int j=0;j<len-1;j++)                mem.push_back(res[i-1][j]+res[i-1][j+1]);            mem.push_back(1);            res.push_back(mem);        }        return res;    }};