Determine if an undirected graph has cycle.
Example 1:
[[1,2],[1,3],[2,3]] -> return trueExample 2:
[[1,2],[2,3],[3,4],[1,5]] -> return falseThis problem asks whether an undirected graph contains a cycle. A standard solution is to build an adjacency list and run DFS while tracking the parent node, or use Union-Find to detect when an edge connects two vertices that are already in the same set. In the first example, [[1,2],[1,3],[2,3]] forms a triangle, so the answer is true. In the second example, [[1,2],[2,3],[3,4],[1,5]] has no cycle, so the answer is false.
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