LeetCode OJ——Pascal's Triangle II

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

杨辉三角2，比杨辉三角要求的空间上限制Could you optimize your algorithm to use only O(k) extra space?其实计算当前行，也只用到前一行了。再前面的没有用。

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
 
        vector<int> onepiece;
        if(rowIndex<0)
            return onepiece;
        //1
        onepiece.push_back(1);
         
        if(rowIndex == 0)
            return onepiece;
        //1
        onepiece.push_back(1);
        if(rowIndex == 1)
            return onepiece;

        //2...
        vector<int> ans(onepiece);
        for(int row = 2;row<= rowIndex;row++)
        {
            onepiece = ans;
            for(int index = 1;index < row;index++)
            {
                ans[index] = onepiece[index - 1]+onepiece[index];
            }
            ans.push_back(1);
        }
        return ans;
    }
};