[LeetCode] Pascal's Triangle

Given numRows, generate the first numRows of Pascal's triangle.

For example, given numRows = 5, Return

[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

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;
    }
};