<p>At a lemonade stand, each lemonade costs <code>$5</code>. 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 <code>$5</code>, <code>$10</code>, or <code>$20</code> bill. You must provide the correct change to each customer so that the net transaction is that the customer pays <code>$5</code>.</p>

<p>Note that you do not have any change in hand at first.</p>

<p>Given an integer array <code>bills</code> where <code>bills[i]</code> is the bill the <code>i<sup>th</sup></code> customer pays, return <code>true</code> <em>if you can provide every customer with the correct change, or</em> <code>false</code> <em>otherwise</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> bills = [5,5,5,10,20]
<strong>Output:</strong> true
<strong>Explanation:</strong> 
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.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> bills = [5,5,10,10,20]
<strong>Output:</strong> false
<strong>Explanation:</strong> 
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.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= bills.length &lt;= 10<sup>5</sup></code></li>
	<li><code>bills[i]</code> is either <code>5</code>, <code>10</code>, or <code>20</code>.</li>
</ul>

class Solution {
public:
    bool lemonadeChange(vector<int>& bills) {
       
        int n=bills.size();
        unordered_map<int,int>mp;

        for(int i=0;i<n;i++){
            
            
            if(bills[i]==10){
                if(mp.find(5)==mp.end()){return false;}
                mp[5]--;
                if(mp[5]==0){
                    mp.erase(5);
                }
                mp[bills[i]]++;
            }
            else if(bills[i]==20){
    
                  if(mp.find(10)!=mp.end() && mp.find(5)!=mp.end() ){
                 
                 mp[10]--;
                if(mp[10]==0){mp.erase(10);}
                    mp[5]--;
                if(mp[5]==0){mp.erase(5);}
                 }
                else if(mp.find(5)!=mp.end()){
                    if(mp[5]>=3){
                        mp[5]-=3;
                        if(mp[5]==0){mp.erase(5);}
                    }
                    else{
                        return false;
                    }
                }
                else{
                    return false;
                }
              
               
               
            }
           else{
            mp[bills[i]]++;
           }
        }
        
        return true;
    }
};
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