leetcode[118]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;
        vector<int> curr;
        if(numRows<1)return res;
        if(numRows==1)
        {
            curr.push_back(1);
            res.push_back(curr);
            return res;
        }
        if(numRows==2)
        {
            curr.push_back(1);
            res.push_back(curr);
            curr.push_back(1);
            res.push_back(curr);
            return res;
        }
        curr.push_back(1);
        res.push_back(curr);
        curr.push_back(1);
        res.push_back(curr);
        curr.clear();
        for(int i=2;i<numRows;i++)
        {
            curr.resize(i+1);
            for(int j=0;j<=i;j++)
            {
                if(j==0||j==i)curr[j]=1;
                else
                {
                    curr[j]=res[i-1][j-1]+res[i-1][j];
                }
            }
            res.push_back(curr);
            curr.clear();
        }
        return res;
    }
};