Pascal's Triangle

class Solution {public:    vector<vector<int> > generate(int numRows) {        vector<vector<int> > ret;if(numRows<1) return ret;for(int i=0;i<numRows;i++){vector<int> unit;for(int j=0;j<=i;j++){if(j==0 || j==i){unit.push_back(1);//两端为1}else{unit.push_back(ret[i-1][j-1]+ret[i-1][j]);}}ret.push_back(unit);}return ret;    }};