119. Pascal's Triangle II

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?

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思路：

       思路与Pascal's Triangle 一样 利用上一层的值改变当前层的值 只是遍历改变值由后往前遍历了 只是空间变为O(k)

C++ 代码:

class Solution {
public:
    vector<int> getRow(int rowIndex) {
        vector<int> res(rowIndex+1,1);
        for(int i=1;i<rowIndex;i++){
            for(int j=i;j>0;j--){
                res[j] += res[j-1];
            }
        }
        return res;
    }
};