Given an array `nums` of length `n`. All elements appear in pairs except one of them. Find this single element that appears alone.\nPairs of the same element cannot be adjacent:\n```\n[2, 2, 1, 2, 2] // ok\n[2, 2, 2, 2, 1] // not allowed\n```\n\n**Example 1:**\n```\nInput: [2, 2, 1, 1, 9, 9, 5, 2, 2]\nOutput: 5\n```\n\n**Example 2:**\n```\nInput: [1, 1, 2]\nOutput: 2\n```\n\n**Example 3:**\n```\nInput: [3, 3, 2, 3, 3]\nOutput: 2\n```\n\n<details> \n  <summary>Follow-up</summary>\n\nCan you do better than O(n)?\n</details>\n\n##\n\n<details> \n  <summary>My solution</summary>\n\n`Java` binary search: https://leetcode.com/playground/PeyK4kLt\nTime complexity: O(logn).\nSpace complexity: O(1).\n</details>

Given an array nums of length n. All elements appear in pairs except one of them. Find this single element that appears alone.\nPairs of the same element cannot