Segmentation Fault when popping from a stack

Question

Anyone can help me with finding out the error with this code? This is for a hackerrank question MAXIMUM ELEMENT. For the case 2, the line "maxes.pop()" keeps giving me segmentation fault. Commenting that line out actually allows the code to compile.

QUESTION:

You have an empty sequence, and you will be given queries. Each query is one of these three types:

1 x -Push the element x into the stack.
2 -Delete the element present at the top of the stack.
3 -Print the maximum element in the stack.

Function Description

Complete the getMax function in the editor below.

getMax has the following parameters:

• string operations[n]: operations as strings

Returns

• int[]: the answers to each type 3 query

Input Format

The first line of input contains an integer, . The next lines each contain an above mentioned query.

Constraints

Constraints

All queries are valid.

Sample Input

STDIN   Function
-----   --------
10      operations[] size n = 10
1 97    operations = ['1 97', '2', '1 20', ....]
2
1 20
2
1 26
1 20
2
3
1 91
3

Sample Output

26
91

    vector<int> getMax(vector<string> operations) {
    
    stack<int> nums;
    stack<int> maxes;
    vector<int> maxnums;
    int max = INT_MIN;
    //int top = -1;
    for(long unsigned int i=0; i<operations.size(); i++){
        switch(operations[i][0]){
        case('1'):
            cout<<"Operation 1"<<endl;
            nums.push(stoi(operations[i].substr(2)));
            if(nums.top() > max){
                max = nums.top();
                maxes.push(max);
            }
            break;
        
        case('2'):
            cout<<"Operation 2"<<endl;
            if(max==nums.top()){
                //cout<<"top element in maxes"<<maxes.top()<<endl;
                maxes.pop();
                max = maxes.top();
            }
            nums.pop();
            break;
        
        case('3'):
            cout<<"Operation 3"<<endl;
            maxnums.push_back(maxes.top());
            break;
        }
    }
    return maxnums;
}

Answer 1

Consider the following input sequence:

1 1 // push 1. Pushes 1 into nums and maxes
1 1 // push 1. Pushes 1 into nums, but not into maxes, since max = 1.
3   // delete top stack element
3   // delete top stack element

Before processing the first 3 line, your state will be this:

nums = {1, 1}
maxes = {1}
max = 1

Now, on the first pop, everything will be fine, so after the first pop, you will end up with this state:

nums = {1}
maxes = {}
max = 1

However, on on the second pop, max == nums.top() is still true, so you pop from your maxes stack, which is already empty. That is why it gives you the segmentation fault.