Nearest Two Words

Problem

A search tool ranks a document higher when two query words appear close together. Given a list of words and two different words a and b that each appear at least once, return the smallest distance between a position holding a and a position holding b. The distance between positions i and j is the absolute value of i minus j.

Examples

Input:  words = ["cat", "dog", "fox", "cat", "owl", "dog"], a = "cat", b = "dog"
Output: 1
Why:    the cat at 0 and the dog at 1 are neighbours

Input:  words = ["red", "sky", "sky", "sky", "blue"], a = "blue", b = "red"
Output: 4
Why:    each word appears once, at opposite ends

Input:  words = ["up", "down"], a = "up", b = "down"
Output: 1
Why:    edge case, the shortest list that can hold both words

Hints

0 / 3

1. Hint 1reveal
2. Hint 2locked
3. Hint 3locked

def nearest_pair(words, a, b):
    last_a = last_b = None           # latest position of each word so far
    best = len(words)
    for i, w in enumerate(words):
        if w == a:
            last_a = i
        elif w == b:
            last_b = i
        else:
            continue                 # other words cannot change the answer
        if last_a is not None and last_b is not None:
            best = min(best, abs(last_a - last_b))
    return best

print(nearest_pair(["cat", "dog", "fox", "cat", "owl", "dog"], "cat", "dog"))  # -> 1
print(nearest_pair(["red", "sky", "sky", "sky", "blue"], "blue", "red"))       # -> 4
print(nearest_pair(["up", "down"], "up", "down"))                             # -> 1