This C++ stack implementation doesn't produce any output

Question

void solve(int n, int a[]) {
  stack<int> s;
  s.push(a[0]);
  for (int i = 1; i < n; i++) {
    if (a[i] < s.top()) {
      while (!s.empty()) {
        cout << s.top() << " ";
        s.pop();
      }
      cout << "\n";
    } else {
      s.push(a[i]);
      cout << "\n";
    }
  }
}

Here n is the size of array a[]. It doesn't produce any output on console.

example input: a[] = {3, 1, 2}

example expected output:

3
2 1

[I got some output](https://wandbox.org/permlink/biMDqByi9SEsdLaU). What is your input? Please post a [Minimal, Reproducible Example](https://stackoverflow.com/help/minimal-reproducible-example). — MikeCAT, May 17 '20 at 01:32
When `a[i] < s.top()`, you empty the stack. On the next iteration, you access `s.top()` whereupon your program exhibits undefined behavior. — Igor Tandetnik, May 17 '20 at 01:42

score 0 · Answer 1 · answered May 17 '20 at 01:42

You accessed s.top() without checking if s is empty at if (a[i] < s.top()) { and it is causing Segmentation Fault.
Extra newlines are printed at the latter cout << "\n";.
Values that are smaller than previous values are dropped.
Last chunk of input won't be printed.

Try this:

void solve(int n , int a[]){
    stack<int> s; 
    s.push(a[0]);  
    for(int i=1;i<n;i++){
        if(!s.empty() && a[i] < s.top()){
            while(!s.empty()){
                cout << s.top() <<" ";
                s.pop();
            }
            cout << "\n";
        }
        s.push(a[i]); 
    }
    while(!s.empty()){
        cout << s.top() <<" ";
        s.pop();
    }
    cout << "\n";
}

Or this:

void flush_stack(stack<int>& s) {
    while(!s.empty()){
        cout << s.top() <<" ";
        s.pop();
    }
    cout << "\n";
}

void solve(int n , int a[]){
    stack<int> s; 
    s.push(a[0]);  
    for(int i=1;i<n;i++){
        if(!s.empty() && a[i] < s.top()){
           flush_stack(s);
        }
        s.push(a[i]); 
    }
    flush_stack(s);
}

This C++ stack implementation doesn't produce any output

1 Answers1