leetcode_pascal_triangle

class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int>> ret;
        if(numRows <= 0)
            return ret;
        ret.push_back({1});
        vector<int> pre_row = ret[0];
        for(int i = 2; i <= numRows; i++)
        {
            vector<int> cur_row(i,1);
            for(int j = 1; j < i-1; j++)
            {
                cur_row[j] = pre_row[j-1]+pre_row[j];
            }
            ret.push_back(cur_row);
            pre_row = cur_row;
        }
        return ret;
    }
};