667. Beautiful Arrangement II

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 = 1Output: [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.

Example 2:

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

Note:

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

返回一个长度为n的数组，要求相邻的數的差值的绝对值有k种。想法是首先前k个数构造成[1, n, 2, n-1....]这种形式，这样前k-1个差值的绝对值就是n-1,n-2,n-3...剩下的数接上剩下的，使的剩下的差值都为1。比如n=6,k=3时，就是1,6,5,4,3,2。

代码：

class Solution {public:    vector<int> constructArray(int n, int k) {        vector<int> ans(n);        int i, l = 1, r = n;        for(i = 1; i <= k; ++i) {            if(i & 1) ans[i-1] = l++;            else ans[i-1] = r--;        }        if(i & 1) {            for(i; i <= n; ++i) {                ans[i-1] = r--;            }        }        else {            for(i; i <= n; ++i) {                ans[i-1] = l++;            }        }        return ans;    }};