667. Beautiful Arrangement II

标签（空格分隔）： leetcode

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1.原题

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.

2.分析：

只需要输出其中一种，所以，最直接的方法就是凑K出来就行。
注意到 1 <= k < n
也就是提示，range(n)一共最多有k种这类的差值，[(1, n), (2, n-1), (3, n-2)……]

3.代码实现过程中的一些坑：

当k是奇数的时候, 在排好k-1个差值的序列后, 接下来的值需要按照升序排
当k是偶数的时候, 在排好k-1个差值的序列后，接下来的值需要按照降序排

n = 5, k = 2
[1,5] + [4, 3, 2]

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n = 5, k = 3
[1,5,2] + [3, 4]

4. 代码

class Solution {
public:
    vector<int> constructArray(int n, int k) {
        vector<int> ans;
        int j = 1;
        for (int i = 1; i <= k; i++){
            if (i%2){
                ans.push_back(j);
                j++;
            }
            else {
                ans.push_back(n - j + 2);
            }
        }
        if (k%2){
            for (int i = k+1; i <= n; i++){
                ans.push_back(j++);
            }
        }
        else {
            j = n-j+1;
            for (int i = k+1; i <= n; i++){
                ans.push_back(j--);
            }
        }
        return ans;
    }
};