119. Pascal's Triangle II

题目来源【Leetcode】

Given an index k, return the kth row of the Pascal’s triangle.

For example, given k = 3,
Return [1,3,3,1].

Note:
Could you optimize your algorithm to use only O(k) extra space?
这道题就是显示杨辉三角行的某一行；

class Solution {public:    vector<int> getRow(int rowIndex) {        vector<vector<int>>tr(rowIndex+1);        for(int i = 0; i <= rowIndex; i++){           tr[i].resize(i+1);           for(int j = 0; j < i+1; j++){              if(j == 0 || j == i) tr[i][j] = 1;              else  tr[i][j] = tr[i-1][j-1]+tr[i-1][j];           }        }        return tr[rowIndex];    }};