输出Pascal's Triangle（杨辉三角）

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]
]

C++做此题，很简单，但由于STL的vector掌握不好，耽误了不少时间，下面是代码：

class Solution {
public:
    vector<vector<int> > generate(int numRows) {

        
        vector<vector<int> > pasTri;
        if(numRows <= 0)
            return pasTri;
        vector<int> firstrow(1,1);
        pasTri.push_back(firstrow);

        for(int numRow = 1; numRow < numRows; numRow++)
        {
            vector<int> row;
            for(int rowi=0; rowi<numRow+1; rowi++)
            {
                if(rowi == 0 || rowi == numRow)
                    row.push_back(1);
                else
                    row.push_back(pasTri[numRow-1][rowi-1] + pasTri[numRow-1][rowi]);
            }
            pasTri.push_back(row);
            
        }
        return pasTri;
    }
};