# Explores the network of words D starting at the source word. If the exploration ends
# without reaching the target word, then there is no path between the source and the target.
# In this case the function returns False. Otherwise, there is a path and the function
# returns True.
def searchWordNetwork(source, target, D):

    # Initialization: processed and reached are two dictionaries that will help in the 
    # exploration. 
    # reached: contains all words that have been reached but not processed.
    # processed: contains all words that have been reached and processed, i.e., their neighbors have also been explored.
    # The values of keys are not useful at this stage of the program and so we use 0 as dummy values.
    processed = {source:0}
    reached = {}
    for e in D[source]:
        reached[e] = source # the value in the dictionary of a key k is the "parent" of k
       
    # Repeat until reached set becomes empty or target is reached 
    while reached:
        # Check if target is in reached; this would imply there is path from source to target
        if target in reached:
            processed.update({target:reached[target]})
            return processed
        
        # Pick an item in reached and process it
        item = reached.popitem() # returns an arbitrary key-value pair as a tuple
        newWord = item[0]
        parent = item[1]
        
        # Find all neighbors of this item and add new neighbors to reached
        processed[newWord] = parent
        for neighbor in D[newWord]:
            if neighbor not in reached and neighbor not in processed:
                reached[neighbor] =  newWord
    
    return {}