First common element from two lists

Question

x = [8,2,3,4,5]
y = [6,3,7,2,1]

How to find out the first common element in two lists (in this case, \"2\") in a concise and elegant way? Any list

Answer 1

Using a for loops with in will result in a O(N^2) complexity, but you can sort y here and use binary search to improve the time complexity to O(NlogN).

def binary_search(lis,num):
    low=0
    high=len(lis)-1
    ret=-1  #return -1 if item is not found
    while low<=high:
        mid=(low+high)//2
        if numlis[mid]:
            low=mid+1
        else:
            ret=mid
            break

    return ret

x = [8,2,3,4,5]
y = [6,3,7,2,1]
y.sort()

for z in x:
    ind=binary_search(y,z)
    if ind!=-1
        print z
        break

output: 2

Using the bisect module to perform the same thing as above:

import bisect

x = [8,2,3,4,5]
y = [6,3,7,2,1]
y.sort()

for z in x:
    ind=bisect.bisect(y,z)-1  #or use `ind=min(bisect.bisect_left(y, z), len(y) - 1)`
    if ind!=-1 and y[ind] ==z:
        print z      #prints 2
        break