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Python Program for Segregate 0s and 1s in an array

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You are given an array of 0s and 1s in random order. Segregate 0s on left side and 1s on right side of the array. Traverse array only once. 

Example:

Input array   =  [0, 1, 0, 1, 0, 0, 1, 1, 1, 0] 
Output array =  [0, 0, 0, 0, 0, 1, 1, 1, 1, 1] 

Method #1: Using sort() function 

Python3




# driver program to test
arr = [0, 1, 0, 1, 1, 1]
arr.sort()
print("Array after segregation is", end=' ')
print(*arr)
 
# This code is contributed by vikkycirus


Output

Array after segregation is 0 0 1 1 1 1

Time Complexity: O(n*logn)
Auxiliary Space: O(1)

Method 2: (Count 0s or 1s) 

  • Count the number of 0s. Let count be C. 
  • Once we have count, we can put C 0s at the beginning and 1s at the remaining n – C positions in array.

Time Complexity : O(n)
Auxiliary Space: O(1)
Thanks to Naveen for suggesting this method. 

Method 3: (Use two indexes to traverse) 
Method 2 traverses the array two times. Method 3 does the same in a single pass.

Maintain two indexes. Initialize first index left as 0 and second index right as n-1.
Do the following while left < right 
a) Keep incrementing index left while there are 0s at it 
b) Keep decrementing index right while there are 1s at it 
c) If left < right then exchange arr[left] and arr[right]

Implementation: 

Python




# Python program to sort a binary array in one pass
 
# Function to put all 0s on left and all 1s on right
def segregate0and1(arr, size):
    # Initialize left and right indexes
    left, right = 0, size-1
     
    while left < right:
        # Increment left index while we see 0 at left
        while arr[left] == 0 and left < right:
            left += 1
 
        # Decrement right index while we see 1 at right
        while arr[right] == 1 and left < right:
            right -= 1
 
        # If left is smaller than right then there is a 1 at left
        # and a 0 at right. Exchange arr[left] and arr[right]
        if left < right:
            arr[left] = 0
            arr[right] = 1
            left += 1
            right -= 1
 
    return arr
 
# driver program to test
arr = [0, 1, 0, 1, 1, 1]
arr_size = len(arr)
print("Array after segregation")
print(segregate0and1(arr, arr_size))
 
# This code is contributed by Pratik Chhajer


Output

Array after segregation
[0, 0, 1, 1, 1, 1]

Time Complexity: O(n)
Auxiliary Space: O(1)

Another approach : 
1. Take two pointer type0(for element 0) starting from beginning (index = 0) and type1(for element 1) starting from end (index = array.length-1). 
Initialize type0 = 0 and type1 = array.length-1 
2. It is intended to Put 1 to the right side of the array. Once it is done, then 0 will definitely towards left side of array.

Please refer complete article on Segregate 0s and 1s in an array for more details!



Last Updated : 17 Mar, 2023
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