【Pascal's Triangle】cpp

class Solution {
public:
    vector<vector<int>> generate(int numRows) {
            vector<vector<int> > ret;
            if ( numRows<1 ) return ret;
            vector<int> pre, curr;
            for ( int i=0; i<numRows; ++i )
            {
                curr.clear();
                curr.push_back(1);
                for ( int j=0; j<pre.size(); ++j )
                {    
                    if ( j==pre.size()-1 ){
                        curr.push_back(1);
                    }
                    else{
                        curr.push_back(pre[j]+pre[j+1]);
                    }
                }
                pre = curr;
                ret.push_back(curr);
            }
            return ret;
    }
};