Given a non-negative integer numRows, generate the first numRows of Pascal's triangle.

 

In Pascal's triangle, each number is the sum of the two numbers directly above it.

Example:

Input: 5
Output:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]
很基础的题目，大学刚开始学C语言或者C++的时候，都会练习的一道题。主要收获就是对二维动态数组有了更多的了解。
方法：定义滑动标签，用于所需计算层的上一层数据获取，来计算 当前层的值。

class Solution {
public:
vector<vector<int>> generate(int numRows) {
    if (numRows < 1)  return{};
    if (numRows == 1) return{ {1} };
    if (numRows == 2) return{ {1},{1,1} };
    vector<vector<int>> result(numRows);
    result[0].push_back(1);
    result[1].push_back(1);
    result[1].push_back(1);
    int counter=2;
    while (counter < numRows) {
        int tag1 = 0, tag2 = 1;
        result[counter].push_back(1);
        while (1)
        {
            result[counter].push_back(result[counter - 1][tag1] + result[counter - 1][tag2]);
            tag1++;
            tag2++;
            if (tag2 >=result[counter-1].size()) {
                result[counter].push_back(1);
                break;
            }
        }
        counter++;
    }
    return result;
  }
};