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860.lemonade-change.cpp
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69 lines (65 loc) · 2.29 KB
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// Tag: Array, Greedy
// Time: O(N)
// Space: O(1)
// Ref: -
// Note: -
// At a lemonade stand, each lemonade costs $5. Customers are standing in a queue to buy from you and order one at a time (in the order specified by bills). Each customer will only buy one lemonade and pay with either a $5, $10, or $20 bill. You must provide the correct change to each customer so that the net transaction is that the customer pays $5.
// Note that you do not have any change in hand at first.
// Given an integer array bills where bills[i] is the bill the ith customer pays, return true if you can provide every customer with the correct change, or false otherwise.
//
// Example 1:
//
// Input: bills = [5,5,5,10,20]
// Output: true
// Explanation:
// From the first 3 customers, we collect three $5 bills in order.
// From the fourth customer, we collect a $10 bill and give back a $5.
// From the fifth customer, we give a $10 bill and a $5 bill.
// Since all customers got correct change, we output true.
//
// Example 2:
//
// Input: bills = [5,5,10,10,20]
// Output: false
// Explanation:
// From the first two customers in order, we collect two $5 bills.
// For the next two customers in order, we collect a $10 bill and give back a $5 bill.
// For the last customer, we can not give the change of $15 back because we only have two $10 bills.
// Since not every customer received the correct change, the answer is false.
//
//
// Constraints:
//
// 1 <= bills.length <= 105
// bills[i] is either 5, 10, or 20.
//
//
class Solution {
public:
bool lemonadeChange(vector<int>& bills) {
int change5 = 0;
int change10 = 0;
for (int b: bills) {
if (b == 5) {
change5 += 1;
} else if (b == 10) {
if (change5 > 0) {
change5 -= 1;
change10 += 1;
} else {
return false;
}
} else if (b == 20) {
if (change10 > 0 && change5 > 0) {
change10 -= 1;
change5 -= 1;
} else if (change5 > 2) {
change5 -= 3;
} else {
return false;
}
}
}
return true;
}
};