[ Debug Help] [Spoj] ADABRANC - Ada and Branches

I am getting a SIGSEGV error, i am unable to find where my code might be accessing invalid memory location.


Got Accepted, instead of passing comparator for sorting, i passed greater<vector<int>>().

Can anyone explain why external comparator function gave SIGSEGV and greater<vector<int>>() did not?

Problem Link

#include <bits/stdc++.h>
using namespace std;

struct node{
    int par;
    int count;
    int rank;

bool comp(vector<int> &a, vector<int> &b){
    return a[0] >= b[0];

node find(vector<node> &parent, int a){
    while(parent[a].par != a){
        a  = parent[a].par;
    return parent[a];

void unionSet(int a, int b, vector<node> &parent){
    node aRoot = find(parent, a);
    node bRoot = find(parent, b);

    if(aRoot.par == bRoot.par)return;

    if(aRoot.rank > bRoot.rank){
        swap(aRoot, bRoot);

    parent[aRoot.par].par = bRoot.par;

    parent[bRoot.par].count += aRoot.count;

    if(aRoot.rank == bRoot.rank){
        parent[bRoot.par].rank += 1;

void solve(){
    int n, m, q;
    vector<node> parent(n);

    for(int i=0;i<n;i++){
        parent[i].par = i;
        parent[i].rank = 0;
        parent[i].count = 1;

    vector<vector<int>> edges(m, vector<int>(3)), queries(q, vector<int>(2));

    for(int i=0;i<m;i++){
        cin>>edges[i][1]>>edges[i][2]>>edges[i][0]; //w,a,b

    for(int i=0;i<q;i++){
        cin>>queries[i][1]>>queries[i][0]; // w, src, index

    sort(edges.begin(), edges.end(), comp);
    sort(queries.begin(), queries.end(), comp);

   // debug2d(edges);

    vector<int> res(q);
    int j = 0;

    for(int i=0;i<q;i++){
        while(j < m && edges[j][0] >= queries[i][0]){
            unionSet(edges[j][1], edges[j][2], parent);
        res[queries[i][2]] = find(parent, queries[i][1]).count;

    for(int i=0;i<q;i++)cout<<res[i]<<endl;

int main(){
		freopen("in.txt", "r", stdin);
		freopen("out.txt", "w", stdout);
    int t;
    t = 1;
    return 0;
Debug output
suman@skynet:~/temp$ c++ -g Wrong.cpp 
suman@skynet:~/temp$ gdb a.out 
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Type "show copying" and "show warranty" for details.
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Reading symbols from a.out...
(gdb) run < in.in > output.out 
Starting program: /home/suman/temp/a.out < in.in > output.out

Program received signal SIGSEGV, Segmentation fault.
comp (a=std::vector of length 0, capacity 0, b=std::vector of length 3, capacity 4 = {...}) at Wrong.cpp:11
11          return a[0] >= b[0];
(gdb) quit
A debugging session is active.

        Inferior 1 [process 216994] will be killed.

Quit anyway? (y or n) y

Focus on this:

Program received signal SIGSEGV, Segmentation fault.
comp (a=std::vector of length 0, capacity 0, b=std::vector of length 3, capacity 4 = {...}) at Wrong.cpp:11
11          return a[0] >= b[0];

It says vector of length 0, but you are trying to access the element at index 0, hence the segmentation fault.

What’s the testcase?

It is randomly generated. I am pasting the generator here. If you need the actual test case which I used, comment about it and I will upload it to my git.

Test case generator - Python code
from random import randint

with open("in.in", "w") as file:
    file.write("100000 200000 300000\n")
    taken = set()
    for i in range(2 * 10**5):
        a, b = 0, 1
        while (a, b) in taken:
            a = randint(0, 10**5 - 1)
            b = randint(0, 10**5 - 1)
            while b == a:
                b = randint(0, 10**5 - 1)
        taken.add((a, b))
        file.write(str(a) + " " + str(b) + "\n")
    for i in range(3 * 10**5):
        a = randint(0, 10**5 - 1)
        y = randint(1, 10**5)
        file.write(str(a) + " " + str(y) + '\n')