LeetCode_pascals-triangle

https://leetcode.com/problems/pascals-triangle/
解题思路：第二行起，上一行的两个数之和作为下一行的一个数；

class Solution {public:    vector<vector<int>> generate(int numRows) {        vector<int> up(2,1);        vector<int> down(1,1);        vector<vector<int>> floor;        if(numRows == 1){            floor.push_back(down);            return floor;        }        if(numRows == 0){            return floor;        }        floor.push_back(down);        floor.push_back(up);            int sum;        for(int i = 2;i < numRows;i++){            for(int j = 1;j < up.size();j++){                sum = up[j] + up[j-1];                down.push_back(sum);            }            down.push_back(1);            floor.push_back(down);            up.assign(down.begin(),down.end());            down.clear();//注意，要清空数组；            down.push_back(1);//返回的原始状态        }        return floor;    }};