技術標籤：leetcode每日一題202101leetcode並查集

Problem

In this problem, a tree is an undirected graph that is connected and has no cycles.

The given input is a graph that started as a tree with N nodes (with distinct values 1, 2, …, N), with one additional edge added. The added edge has two different vertices chosen from 1 to N, and was not an edge that already existed.

The resulting graph is given as a 2D-array of edges. Each element of edges is a pair [u, v] with u < v, that represents an undirected edge connecting nodes u and v.

Return an edge that can be removed so that the resulting graph is a tree of N nodes. If there are multiple answers, return the answer that occurs last in the given 2D-array. The answer edge [u, v] should be in the same format, with u < v.

Note:

• The size of the input 2D-array will be between 3 and 1000.
• Every integer represented in the 2D-array will be between 1 and N, where N is the size of the input array.

Example1

Example2

Solution

class Solution {
public:

    vector<int> p;

    int find(int x) {
        if (p[x] != x) p[x] =
 
 find(p[x]);
        return p[x];
    }

    vector<int> findRedundantConnection(vector<vector<int>>& edges) {
        if(edges.empty())
            return {};
        
        int N = 0;
        for(int i = 0;i<edges.size();++i)
        {
            int u = edges[i][0];
            int v = edges[i][1];

            N = max(N,max(u,v));
        }

        for(int i = 0;i<=N;++i)
            p.push_back(i);

        for(int i = 0;i<edges.size();++i)
        {
            int u = edges[i][0];
            int v = edges[i][1];

            if(find(u) != find(v))
                p[find(u)] = find(v);
            else
                return {u,v};
        }

        return {};

    }
};