重新排序一个序列

question：



Given an unsorted array nums, reorder it such that nums[0] < nums[1] > nums[2] < nums[3]....

Example:
     Given nums = [1, 5, 1, 1, 6, 4], one possible answer is [1, 4, 1, 5, 1, 6]. 
   

my solution：

void wiggleSort(vector<int>& nums){
if (nums.empty()) {
return;
}   
int n = nums.size();

// Step 1: Find the median
vector<int>::iterator nth = next(nums.begin(), n /2);
nth_element(nums.begin(), nth, nums.end());
int median = *nth;

// Step 2: Tripartie partition within O(n)-time & O(1)-space.
auto m = [n](int idx) {return (2 * idx +1) % (n | 1); };   
int first = 0, mid =0, last = n - 1;
while (mid <= last) {
if (nums[m(mid)] > median) {
swap(nums[m(first)], nums[m(mid)]);
++first;
++mid;
}
else if (nums[m(mid)] < median) {
swap(nums[m(mid)], nums[m(last)]);
--last;
}
else {
++mid;
}
}
}