Pascal's Triangle

理清思路，还是很简单的。从上往下依次构建即可。

class Solution {public:    vector<vector<int> > generate(int numRows) {        vector<vector<int>> res;        if (numRows == 0)            return res;        res.push_back(vector<int>(1, 1));        if (numRows == 1)            return res;        res.push_back(vector<int>(2, 1));        if (numRows == 2)            return res;                vector<int> tmp;        for (int i = 3; i <= numRows; ++i) {            tmp.clear();            tmp.push_back(1);            vector<int> last = res[res.size() - 1];            for (int j = 0; j < i - 2; ++j) {                tmp.push_back(last[j] + last[j + 1]);            }            tmp.push_back(1);            res.push_back(tmp);        }        return res;    }};

http://oj.leetcode.com/problems/pascals-triangle/