# Task
 Some people are standing in a row in a park. There are trees between them which cannot be moved. 
 Your task is to rearrange the people by their heights in a non-descending order without moving the trees.
# Example
 For `a = [-1, 150, 190, 170, -1, -1, 160, 180]`, the output should be
 `[-1, 150, 160, 170, -1, -1, 180, 190]`.
# Input/Output
 - `[input]` integer array `a`
    If a[i] = -1, then the ith position is occupied by a tree. Otherwise a[i] is the height of a person standing in the ith position.
    Constraints:
    `5 ≤ a.length ≤ 30,`
    `-1 ≤ a[i] ≤ 200.`
 - `[output]` an integer array
    `Sorted` array a with all the trees untouched.
def sort_by_height(a):
    h, k = sorted([i for i in a if i!=-1]), []
    for i in a:
        if i!=-1:
            k+=[h[0]]
            h=h[1:]
        else:
            k+=[i]
    return k
def sort_by_height(a):
    h, k = sorted([i for i in a if i!=-1]), []
    for i in a:
        if i!=-1:
            k+=[h[0]]
            h=h[1:]
        else:
            k+=[i]
    return k