【LeetCode】Pascal's Triangle

class Solution 
{
public:
    vector<vector<int> > generate(int numRows) 
    {
        vector<vector<int> > result;
        if(numRows == 0)
        {
            //nothing
        }
        else if(numRows == 1)
        {
            //第0层
            vector<int> cur;
            cur.push_back(1);
            result.push_back(cur);
        }
        else
        {
            //第0层
            vector<int> cur;
            cur.push_back(1);
            result.push_back(cur);

            for(int n = 1; n < numRows; n ++)
            {//用<n choose m>的通项公式来做肯定溢出，所以只能用加法做
                vector<int> cur;
                //头
                cur.push_back(1);
                //中间元素是上层两元素相加
                for(int i = 0, j = 1; j < n; i++, j++)
                {
                    vector<int> pre = result[result.size()-1];
                    cur.push_back(pre[i]+pre[j]);
                }
                //尾
                cur.push_back(1);
                result.push_back(cur);
            }
        }
        return result;
    }
};