Lemonade - Algorithm

Keep count of $5 and$10 bills you have:

$1.$ Customer pays $5: take it (fives += 1)

$2.$ Customer pays $10: give one$5 back if possible (fives -= 1, tens += 1)

$3.$ Customer pays $20: prefer giving one$10 + one $5; else give three$5s If you ever cannot give correct change, return false. No need to track $20 bills since you never give them as change. Time:$O(n \log n)$. Space:$O(1)$. The time complexity is$O(n)$, not$O(n log n)$, because you do not need to sort. You process bills in the order customers arrive.