merge two arrays without extra space

class Solution:
    def merge(self,a,b,m,n):
        nex=lambda x:x//2+(x%2) if x>1 else 0 #This is very important
        gap=nex(m+n)
        def swap(x,y,a,b):
            a[x],b[y]=b[y],a[x]

        while gap:
            i,j=0,gap
            while j<m+n:
                if i<m and j<m:
                    if a[i]>a[j]:swap(i,j,a,a)
                elif i<m and j>=m:
                    if a[i]>b[j-m]:swap(i,j-m,a,b)
                elif i>=m and j>=m:
                    if b[i-m]>b[j-m]:swap(i-m,j-m,b,b)
                i,j=i+1,j+1
            gap=nex(gap)