想到的：
这k个差的取值最大是n-1，最小是1
解题思路：
class Solution {
public int[] constructArray(int n, int k) {
if( k >= n )return null;
int[] res = new int[n];
int pre = 1;
int rear = n;
int i = 0;
while( i < k ){
res[ i ++ ] = pre++;
if( i < k )res[ i ++ ] = rear--;
}
if( k % 2 == 0 ){
while( i < n )
res[i ++ ] = rear --;
}
if( i % 2 == 1 ){
while( i < n )
res[ i ++ ] = pre ++;
}
return res;
}
}