Key cutting

#include<iostream>
#include<vector>
typedef long long ll;
using namespace std;
 
void decrease(vector<ll>& key, ll a, ll b, ll minim){
    for(ll i=a; i<b; i++){
        key[i] -= minim;
    }
}

ll calc(int a, int b, vector<ll>& key){

    ll start = -1;
    ll minim;
 
    ll op = 0;
    for(ll i = a; i < b; i++){
        if(start == -1 and key[i]!=0){
            start = i;
            minim = key[i];
        }

        if(start != -1 and key[i]!=0 and key[i]<minim)minim = key[i];
        
        if(start != -1 and (key[i]==0 or i==b-1)){
            if(key[i]==0 ){
                decrease(key,start,i,minim);
                op += calc(start,i,key); 
            }
            else{
                decrease(key,start,i+1,minim);
                op += calc(start,i+1,key); 
            }

            op +=1;

            // for(int l=0; l<key.size(); l++){
            //     cout << key[l] << " ";
            // }
            // cout << endl;
            // ll lala=0;
            // while(lala!=1000000)lala+=1;

            
            start = -1;
            

        }

    }
    return op;
}
 
int main(){
    ll n;
    cin >> n;
    vector<ll> key(n);
 
    for(ll i = 0; i < n; ++i) cin >> key[i];

    cout << calc(0,n,key)<<endl;
 
    
    
 
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED