Leetcode 667. Beautiful Arrangement II

###題目 連接：https://leetcode.com/problems/beautiful-arrangement-ii/

**Level: ** Medium

Discription: Given two integers n and k, you need to construct a list which contains n different positive integers ranging from 1 to n and obeys the following requirement: Suppose this list is [a1, a2, a3, ... , an], then the list [|a1 - a2|, |a2 - a3|, |a3 - a4|, ... , |an-1 - an|] has exactly k distinct integers.

If there are multiple answers, print any of them.

Example 1:

Input: n = 3, k = 1
Output: [1, 2, 3]
Explanation: The [1, 2, 3] has three different positive integers ranging from 1 to 3, and the [1, 1] has exactly 1 distinct integer: 1.

Note:

• The n and k are in the range 1 <= k < n <= 104.

###代碼

class Solution {
public:
    vector<int> constructArray(int n, int k) {
        vector<int> ret;
        ret.push_back(1);
        int num=1;
        for(int i=k;i>=1;i--)
        {
            ret.push_back(ret.back()+i*num);
            num*=-1;
        }
        for(int i=k+2;i<=n;i++)
        {
             ret.push_back(i);
        }
        return ret;
    }
};

###思考

• 算法時間複雜度爲O(N),空間複雜度爲O(1)，不算輸出所佔空間。
• 這題比較簡單，找規律，只須要一種序列便可。