[leetcode] Pascal's Triangle

From : https://leetcode.com/problems/pascals-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;
        if(numRows <= 0) return res;
        int i=1;
        vector<int> pre;
        pre.push_back(1);
        res.push_back(pre);
        while(i < numRows) {
            pre = res[i-1];
            vector<int> cur;
            for(int j=0; j<=i; j++) {
                if(j==0 || j==i)  cur.push_back(1);
                else {
                    cur.push_back(pre[j-1] + pre[j]);
                }
            }
			res.push_back(cur);
            i++;
        }
        return res;
    }
};