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?

思路：

最开始的想法是用两个vector依次记录当前行和上一行的结果。但是要求用k的空间，所以使用一个vector，每次从后往前r[j] = r[j] + r[j-1]即可。

代码：

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> result(rowIndex+1,0);
result[0] = 1;
int i, j;
for(i = 1;  i <= rowIndex; i++){
for(j = i; j > 0; j--){
result[j] = result[j] + result[j-1];
}
}
return result;
}