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"title": "Count Unreachable Pairs of Nodes in an Undirected Graph",
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"content": "<p>You are given an integer <code>n</code>. There is an <strong>undirected</strong> graph with <code>n</code> nodes, numbered from <code>0</code> to <code>n - 1</code>. You are given a 2D integer array <code>edges</code> where <code>edges[i] = [a<sub>i</sub>, b<sub>i</sub>]</code> denotes that there exists an <strong>undirected</strong> edge connecting nodes <code>a<sub>i</sub></code> and <code>b<sub>i</sub></code>.</p>\n\n<p>Return <em>the <strong>number of pairs</strong> of different nodes that are <strong>unreachable</strong> from each other</em>.</p>\n\n<p> </p>\n<p><strong>Example 1:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2022/05/05/tc-3.png\" style=\"width: 267px; height: 169px;\" />\n<pre>\n<strong>Input:</strong> n = 3, edges = [[0,1],[0,2],[1,2]]\n<strong>Output:</strong> 0\n<strong>Explanation:</strong> There are no pairs of nodes that are unreachable from each other. Therefore, we return 0.\n</pre>\n\n<p><strong>Example 2:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2022/05/05/tc-2.png\" style=\"width: 295px; height: 269px;\" />\n<pre>\n<strong>Input:</strong> n = 7, edges = [[0,2],[0,5],[2,4],[1,6],[5,4]]\n<strong>Output:</strong> 14\n<strong>Explanation:</strong> There are 14 pairs of nodes that are unreachable from each other:\n[[0,1],[0,3],[0,6],[1,2],[1,3],[1,4],[1,5],[2,3],[2,6],[3,4],[3,5],[3,6],[4,6],[5,6]].\nTherefore, we return 14.\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li><code>1 <= n <= 10<sup>5</sup></code></li>\n\t<li><code>0 <= edges.length <= 2 * 10<sup>5</sup></code></li>\n\t<li><code>edges[i].length == 2</code></li>\n\t<li><code>0 <= a<sub>i</sub>, b<sub>i</sub> < n</code></li>\n\t<li><code>a<sub>i</sub> != b<sub>i</sub></code></li>\n\t<li>There are no repeated edges.</li>\n</ul>\n",
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"hints": [
"Find the connected components of the graph. To find connected components, you can use Union Find (Disjoint Sets), BFS, or DFS.",
"For a node u, the number of nodes that are unreachable from u is the number of nodes that are not in the same connected component as u.",
"The number of unreachable nodes from node u will be the same for the number of nodes that are unreachable from node v if nodes u and v belong to the same connected component."
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