Count the number of 0s , 1s , 2s in the array .
Store all the 0s from the beginning , next store all 1s and then store all the 2s at the end.

Python Code

# Function to sort the array of 0s, 1s and 2s 
def sortArr(arr, n): 
  c0,c1,c2 = 0,0,0
  # Count the number of 0s, 1s and 2s in the array 
    for i in range(n): 
      if arr[i] == 0: 
        c0+=1
          
      elif arr[i] == 1: 
        c1+=1
              
      elif arr[i] == 2: 
        c2+=1
      
      # Update the array 
      i = 0
      
      # Store the 0s in the beginning 
      while (c0 > 0): 
        arr[i] = 0
        i+=1
        c0-=1
      
    # store all the 1s 
    while (c1 > 0): 
        arr[i] = 1
        i+=1
        c1-=1
      
    # store all the 2s 
    while (c2 > 0): 
        arr[i] = 2
        i+=1
        c2-=1
      
    return arr
  
  
# Driver's code 
  
arr = [0,1,2,0,1,1,0,0,2,1,0,2] 
n = len(arr) 
  
print(sortArr(arr, n))