Minimum transactions

gistfile1.py

import collections

# input: list of transactions in form (a, b, amount) corresponding to "a owes b amount"
def min_transactions(transactions):
    # build map of users to change in wealth
    wealth = collections.defaultdict(int)
    for lendee, lender, amount in transactions:
        wealth[lendee] -= amount
        wealth[lender] += amount

    payers = 0
    receivers = 0
    for amount in wealth.itervalues():
        if amount > 0:
            receivers += 1
        elif amount < 0:
            payers += 1

    return max(payers, receivers)