Leetcode: Pascal's Triangle

题目：

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]]

杨辉三角相信是大家再熟悉不过的啦，这里就不分析思路了！

这道题是目前为止，我第一次一遍写好，一次性通过的题目，这个记录下！

下面给出示例代码：

class Solution {public:    vector<vector<int> > generate(int numRows)    {    vector<vector<int>> result;    if (numRows < 1)    {    return result;    }        vector<int> first;    first.push_back(1);    result.push_back(first);    if (numRows == 1)    {    return result;    }        vector<int> second;    second.push_back(1);    second.push_back(1);    result.push_back(second);    if (numRows == 2)    {    return result;    }        result.reserve(numRows);    for (size_t i = 2; i < numRows; i++)    {    vector<int> current;    current.push_back(1);    vector<int> previous(result.back());    for (size_t i = 1; i < previous.size(); i++)    {    current.push_back(previous[i - 1] + previous[i]);    }    current.push_back(1);    result.push_back(current);    }        return result;    }};