链接节点列表，分段FAULT

Question

I'd to create a list of linked nodes. The idea is separate a word by letters, and creating a list of linked nodes by every different letter. Also I try to add the last node in the last position.

I've tried a lot of things, Theorically it works, but I don't find where the segmentation fault origins.

    struct nodo{
        int frecuencia;
        nodo *next;
        char letra;
};

int main(){

    string recibir; cin>>recibir; // recibir is the input.
    nodo *n;
    for(size_t i=0;i< recibir.size();i++){
    agregarnodoalista(n,recibir[i]);}

Add node to list Function:(If there is exist a node with the same letter, add +1 to the frequence).

void agregarnodoalista(nodo *&n, char caracter){
if(buscarletra(n,caracter)) localizarnodo(n,caracter)->frecuencia++;
    else{
    nodo *nodito;
    nodito= new nodo;
    nodo *ptr;
    while(ptr->next!=NULL){
            ptr=ptr->next;
    }
    ptr->next = nodito;
    nodito->letra=caracter;
    }}

Find letter in list and says if it is there or not. function:

bool buscarletra(nodo *n, char letrita){
    nodo *ptr;
    for(ptr=n; ptr!=NULL;ptr=ptr->next){
            if(ptr->letra==letrita) return true;}   
    return false;}

Returns the pointer's node thar it has the letter.

nodo *localizarnodo(nodo *n, char letrita){
    nodo *ptr;
    for(ptr=n; ptr!=NULL; ptr=ptr->next){
            if(ptr->letra==letrita) return ptr;
    }
    return NULL;}

Could you give me some help?

Answer 1

As mentioned in the comments you are accessing uninitialized pointers which is undefined behavior. This code should work but you have memory leaks . I didn't check for other potential issues you might have.

#include <iostream>
#include <string>
using namespace std;

struct nodo {
    int frecuencia;
    nodo *next;
    char letra;
};

bool buscarletra(nodo* n, char letrita) {
    nodo *ptr = n;
    for (; ptr != NULL; ptr = ptr->next) {
        if (ptr->letra == letrita) 
            return true;
    }
    return false;
}

nodo *localizarnodo(nodo *n, char letrita) {
    nodo *ptr;
    for (ptr = n; ptr != NULL; ptr = ptr->next) {
        if (ptr->letra == letrita) return ptr;
    }
    return NULL;
}

void agregarnodoalista(nodo *n, char caracter) {
    if (buscarletra(n, caracter)) localizarnodo(n, caracter)->frecuencia++;
    else {
        nodo *nodito;
        nodito = new nodo;
        nodo *ptr = new nodo;
        while (ptr->next != NULL) {
            ptr = ptr->next;
        }
        ptr->next = nodito;
        nodito->letra = caracter;
    }
}

int main() {

    string recibir; 
    cin >> recibir; // recibir is the input.
    nodo* n = new nodo;
    for (size_t i = 0; i < recibir.size() - 1; i++) {
        n->letra = recibir[i];
        nodo* next = new nodo;
        n->next = next;
        n = n->next;
    }
    n->letra = recibir[recibir.size() - 1];
    n->next = NULL;
    for (size_t i = 0; i < recibir.size(); i++) {
        agregarnodoalista(n, recibir[i]);
    }
}