give you an array a, now you need to rearrange its sequence, such that a[i]>a[i-1] && a[i]>a[i+1] for i=1,3,5,...,

give you an array a, now you need to rearrange its sequence, such that a[i]>a[i-1] && a[i]>a[i+1] for i=1,3,5,...,
Note there might be many sequence of equal elements. of course it may be not able to be rearranged like that.

O(n) time and O(1) space

for(i = 1; i < n; i += 2) //this cannot handle 3 equal numbers consequential 
{
  if((a[i-1] == a[i]) && (a[i] == a[i+1]))
      continue;
  maximum = max3(a[i-1],a[i],a[i+1]);
  if(maximum == a[i])
      continue;
  else if(maximum == a[i-1])
      swap(input[i-1],input[i]);
  else
      swap(input[i],input[i+1]);
}