• Source 
    1. #include <iostream>
    2. #include <stack>
    3. #include <vector>
    4. #include <math.h>
    5. using namespace std;
    6.  
    7. int main ()
    8. {
    9. 	int n;
    10. 	cin>>n;
    11. 	vector <long> arr;
    12. 	vector <long> left (1000005, 0);
    13. 	vector <long> right (1000005, 0);
    14. 	arr.push_back(0);
    15. 	for (int i=1; i<=n; i++)
    16. 	{
    17. 		long tmp;
    18. 		cin>>tmp;
    19. 		arr.push_back (tmp);
    20. 	}
    21. 	arr.push_back(0);
    22.  
    23. 	stack <int> SL;
    24. 	SL.push(0);
    25. 	for (int i=1; i<=n; i++)
    26. 	{
    27. 		while (!SL.empty() && arr[SL.top()]>=arr[i]) SL.pop();
    28. 		if (!SL.empty()) left[i]=SL.top();
    29. 		SL.push(i);
    30. 	}
    31.  
    32. 	stack <int> SR;
    33. 	SR.push(n+1);
    34. 	for (int i=n; i>=1; i--)
    35. 	{
    36. 		while (!SR.empty() && arr[SR.top()]>=arr[i]) SR.pop();
    37. 		if (!SR.empty()) right[i]=SR.top();
    38. 		SR.push(i);
    39. 	}
    40. 	long Ans=0;
    41. 	for (int i=1; i<=n; i++)
    42. 	{
    43. 		if (right[i]-left[i]-1 >= arr[i]) Ans = max (Ans, arr[i]);
    44. 	}
    45. 	cout<<Ans;
    46. 	return 0;
    47. }