Three way partitioning of array around a given value dutch national flag varient

#User function template for Python

class Solution:
    #Function to partition the array around the range such
    #that array is divided into three parts.
    def threeWayPartition(self, arr, a, b):
        # code here
        l=0
        r=len(arr)-1
        m=0
        '''
        l: before l all elements <a
        r:after r all elements >b.
        Actually whenevet we find arr[m]<a
        we directly exchange with arr[l] and
        increment both.
        But in the case of arr[m]>b
        even though after exchanging
        arr[m] and arr[r] we shouldnot
        increment m (we can decrement r).
        Because the place at arr[m] after
        exchanging might be a value <a.
        so we have to stay at m and
        shouldnt increment m immediately
         please check sorting colors video
         neetcode video.
        '''

        while m<=r:#VV imp m and r should not cross
            if arr[m]<a:
                arr[l],arr[m]=arr[m],arr[l]
                l=l+1
            elif arr[m]>b:
                arr[r],arr[m]=arr[m],arr[r]
                m=m-1
                r=r-1
            m=m+1