Find center of star graph

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C++

graph

There is an undirected star graph consisting of n nodes labeled from 1 to n. A star graph is a graph where there is one center node and exactly n - 1 edges that connect the center node with every other node. You are given a 2D integer array edges where each edges[i] = [ui, vi] indicates that there is an edge between the nodes ui and vi. Return the center of the given star graph.
Input: edges = [[1,2],[2,3],[4,2]]
Output: 2
Input: edges = [[1,2],[5,1],[1,3],[1,4]]
Output: 1

What I have tried:

I have tried to count the frequency of each element and whosr frequency count equals n-1 return that value because there is only one node which has n-1 edges. The logic seems to be fine but I'm having problem in implementation part and I'm getting runtime error. Here is what I code -

int findCenter(vector<vector<int>>& edges) {
        int n = edges.size();
        vector<int>ans(n);
        for(int i=0;i<n;i++){
            int a = edges[i][0];
            int b = edges[i][1];
            ans[a]++;
            ans[b]++;
           
        }
        for(int i=0;i<n;i++){
            if(ans[i] > 1){
                return i+1;
            }
        }
        return -1;
    }

When I debug the code I'm getting runtime error bcz of these 2 lines-
ans[a]++ and ans[b]++.
I don't know what is wrong with these two lines but when I comment these 2 lines I don't have any runtime error although the output is incorrect