leetcode練習 Redundant Connection

這次是檢查冗餘的，題目主要練習無向圖中是否有環的判斷。
題目如下：
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.

Example 1:

Input: [[1,2], [1,3], [2,3]]
Output: [2,3]
Explanation: The given undirected graph will be like this:
  1
 / \
2 - 3

Example 2:

Input: [[1,2], [2,3], [3,4], [1,4], [1,5]]
Output: [1,4]
Explanation: The given undirected graph will be like this:
5 - 1 - 2
    |   |
    4 - 3

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.

Update (2017-09-26):
We have overhauled the problem description + test cases and specified clearly the graph is an undirected graph. For the directed graph follow up please see Redundant Connection II). We apologize for any inconvenience caused.

代碼如下：

class Solution {
public:
    vector<int> findRedundantConnection(vector<vector<int>>& edges) {
        unordered_map<int, unordered_set<int>> m;
        for (auto x : edges) {
            if (ifCycle(x[0], x[1], m, -1)) return x;
            m[x[0]].insert(x[1]);
            m[x[1]].insert(x[0]);
        }
        return {};
    }

    bool ifCycle(int cur, int des, unordered_map<int, unordered_set<int>>&m, int pre) {
        if (m[cur].count(des)) return true;
        for (int num : m[cur]) {
            if (num == pre) continue;
            if (ifCycle(num, des, m, cur)) return true;
        }
        return false;
    }

};