give 2 ascending sorted arrays with same length n, find the nth number between these 2 arrays

NthNum.py

#!/usr/bin/python

def get_nth_num(a, b):
    if len(a) != len(b) or len(a) == 0:
        return None
    id_a, id_b = 0, 0
    for _ in xrange(len(a)):
        if a[id_a] <= b[id_b]:
            tmp = a[id_a]
            id_a += 1
        else:
            tmp = b[id_b]
            id_b += 1
    return tmp

def get_nth_num_fast_(a, left_a, right_a, b, left_b, right_b):
    if right_a == left_a:
        return a[left_a] < b[left_b] and a[left_a] or b[left_b]
    mid_a = (left_a + right_a) / 2
    mid_b = (left_b + right_b) / 2
    if (right_a - left_a + 1) % 2 == 0:
        if a[mid_a] < b[mid_b]:
            return get_nth_num_fast_(a, mid_a + 1, right_a, b, left_b, mid_b)
        else:
            return get_nth_num_fast_(a, left_a, mid_a, b, mid_b + 1, right_b)
    else:
        if a[mid_a] < b[mid_b]:
            return get_nth_num_fast_(a, mid_a, right_a, b, left_b, mid_b)
        else:
            return get_nth_num_fast_(a, left_a, mid_a, b, mid_b, right_b)

def get_nth_num_fast(a, b):
    if len(a) != len(b) or len(a) == 0:
        return None
    return get_nth_num_fast_(a, 0, len(a) - 1, b, 0, len(b) - 1)

def main():
    a = [1, 5, 8, 90, 100, 101]
    b = [2, 6, 8, 10, 23, 45]
    print get_nth_num(a, b)
    print get_nth_num_fast(a, b)
    a = [1, 5, 8, 9, 15, 21, 32]
    b = [2, 6, 8, 10, 23, 45, 46]
    print get_nth_num(a, b)
    print get_nth_num_fast(a, b)


if __name__ == '__main__':
    main()