Move all zeros present in an array to the end

Given an array of integers, move all zeros present in the array to the end. The solution should maintain the relative order of items in the array.

Example:

Example:
Input : [1,3,0,0,4,0,9]

Modify array so that it becomes :[1 3 4 9 0 0 0] 

Constraint1 : You are not suppose to use any 
extra space
Constraint2 : You need to change the same array 
with single traversal with O(n) time complexity

Problem level: Easy