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.

思路：果然不能直接DFS，会TLE
 * 就是要想办法凑出k个来
 * 注意到：1..n最多可以弄出n-1个不同的差，比如1..9就是
 * 1 9 2 8 3 7 4 6 5
 * diff: 8 7 6 5 4 3 2 1
 * 是大小交替的，这样的话，只要先凑出k-1个，后面按照顺序来产生1就好了

注意下标控制

class Solution {    public int[] constructArray(int n, int k) {    int[] ret = new int[n];    int l = 1, r = n, i = 0;    for(; i<k; i++) {    if(i % 2 == 0)ret[i] = l++;    elseret[i] = r--;    }        if(i % 2 == 1)    for(int j=l; j<=r; j++)ret[i++] = j;    else     for(int j=r; j>=l; j--)ret[i++] = j;        return ret;    }}