CODEWITHSUNDEEP
c++stack
era s'q
era s'q

Reputation: 591

Stack pop giving segment fault

The stack which I'm using is not empty and I did a check for that still getting segment fault. Let s="aaabbaaccd"

string removePair(string s){
  stack<char>st;
  st.push(s[0]);                  //Doing this,So that first pop without insert doesn't cause error.
  string output="";
  for(int i=1;i<s.length();i++){
    if(st.top()!=s[i])
        st.push(s[i]);
    else if(st.top()==s[i] && st.empty()==false){
        st.pop();               //Gives Segment fault.
    }
  }
  while(st.empty()==false){
    output+=st.top();
    st.pop();
  }
  reverse(output.begin(),output.end());
  return output;
}

Check Full Code Here

Upvotes: 0

Views: 95

Answers (1)

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122350

Here

if(st.top()!=s[i])
    st.push(s[i]);
else if(st.top()==s[i] && st.empty()==false){

You check if the stack is empty, but you do that too late. At the point you call st.empty() you already called st.top() twice. You need to check if there is an element before you try to access it. I am not sure about the logic of the code, but I suppose in the first condition you want to push into an empty stack, otherwise check if s[i] equals top:

if(st.empty() || st.top()!=s[i])
    st.push(s[i]);
else if(!st.empty() && st.top()==s[i]){

Note that both || and && are short-circuiting. Hence in A || B if A == true then B will not be evaluated. Similar in C && D, if C == false then D will not be evaluated.

Upvotes: 3

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