118. Pascal's Triangle

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]
]

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class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int>> v;
        if(numRows==0)return v;
        vector<int> t1(1,1);
        v.push_back(t1);
        for(int i=2;i<=numRows;i++)
        {
            t1.push_back(0);
            vector<int> t2=t1;
            for(int j=1;j<i;j++)
                t2[j]=t1[j]+t1[j-1];
            v.push_back(t2);
            t1=t2;
        }
        return v;
    }
};