LeetCode119. Pascal‘s Triangle II

一、题目

Given an integer rowIndex, return the rowIndexth (0-indexed) row of the Pascal’s triangle.

In Pascal’s triangle, each number is the sum of the two numbers directly above it as shown:

Example 1:

Input: rowIndex = 3
Output: [1,3,3,1]
Example 2:

Input: rowIndex = 0
Output: [1]
Example 3:

Input: rowIndex = 1
Output: [1,1]

Constraints:

0 <= rowIndex <= 33

Follow up: Could you optimize your algorithm to use only O(rowIndex) extra space?

二、题解

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