Algorithms
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      • Intersection of Two Arrays
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      • Reverse Words in a String III
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      • Remove Linked List Elements
      • Odd Even Linked List
      • Design Doubly Linked List
      • Flatten a Multilevel Doubly Linked List
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      • Palindrome Linked List
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      • Intersection of Two Arrays
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      • Top K Frequent Elements
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      • Count Univalue Subtrees
      • Construct Binary Tree from Inorder and Postorder Traversal
      • Construct Binary Tree from Preorder and Inorder Traversal
      • Populating Next Right Pointers in Each Node
      • Lowest Common Ancestor of a Binary Tree
      • Serialize and Deserialize Binary Tree
      • Median of Two Sorted Arrays
      • Invert Binary Tree
      • Find K-th Smallest Pair Distance
      • Split Array Largest Sum
    • Heap
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      • Validate Binary Search Tree
      • Inorder Successor in BST
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      • Search in a Binary Search Tree
      • Insert into a Binary Search Tree
      • Delete Node in a BST
      • Kth Largest Element in a Stream
      • Lowest Common Ancestor of a Binary Search Tree
      • Contains Duplicate III
      • Height-Balanced BST
        • Balanced Binary Tree
        • Convert Sorted Array to Binary Search Tree
    • Map
    • N-ary Tree
      • N-ary Tree Preorder Traversal
      • N-ary Tree Postorder Traversal
      • N-ary Tree Level Order Traversal
      • Maximum Depth of N-ary Tree
      • Encode N-ary Tree to Binary Tree
      • Serialize and Deserialize N-ary Tree
    • Trie
      • Implement Trie (Prefix Tree)
      • Map Sum Pairs
      • Replace Words
      • Design Search Autocomplete System
      • Maximum XOR of Two Numbers in an Array
      • Add and Search Word - Data structure design
      • Word Search II
      • Word Squares
      • Longest Common Prefix
      • Palindrome Pairs
    • Balanced Tree
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      • Red-black Tree
      • AVL Tree
    • Graph
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      • Breadth First Search
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      • Dijkstra Algorithm
  • Sequences
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  • Dynamic Programming
    • Knapsack problem
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    • Maximum Subarray
    • House Robber
  • Interviews
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      • Max Consecutive Ones II
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      • Move Zeroes
      • Remove Duplicates from Sorted Array
      • Merge k Sorted Lists
      • Insert into a Cyclic Sorted List
      • Evaluate Division
      • Inorder Successor in BST
      • Robot Room Cleaner
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      • Course Schedule
      • Validate Binary Search Tree
      • Closest Binary Search Tree Value
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      • Shortest Distance from All Buildings
      • Find K-th Smallest Pair Distance
      • Find K Pairs with Smallest Sums
      • Range Module
      • Insert Interval
      • Sort Transformed Array
      • Merge Intervals
      • Longest Palindromic Substring
      • Next Greater Element I
      • Pacific Atlantic Water Flow
      • Evaluate Reverse Polish Notation
      • Decode Ways
      • Word Break
      • Sentence Screen Fitting
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      • House Robber II
      • Moving Average from Data Stream
      • Peeking Iterator
      • Binary Search Tree Iterator
      • Zigzag Iterator
      • Design Tic-Tac-Toe
      • Range Sum Query 2D - Mutable
      • UTF-8 Validation
      • Maximum Product of Word Lengths
  • Other
    • Game of Life
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  1. Data Structures
  2. Set

Single Number

Given a non-empty array of integers, every element appears _twice _except for one. Find that single one.

Note:

Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

Example 1:

Input: [2,2,1]
Output: 1

Example 2:

Input: [4,1,2,1,2]
Output: 4

Here are couple of solutions.

class Solution {
    public int singleNumber(int[] nums) {
        // 0. add every number and remove it if found, the single number will remain in the set
        Set<Integer> set = new HashSet<>();
        for (int i : nums) {
            if (set.contains(i)) {
                set.remove(i);
            } else {
                set.add(i);
            }
        }
        return set.iterator().next();

        // 1. sort, if the following number is not equal to the current number, that number is the single
//         Arrays.sort(nums); // n log n
//         for (int i = 0; i < nums.length; i += 2) { // 0 2 4 // n / 2
//             if (i + 1 >= nums.length) { // 5 (0, 1, 2, 3, 4)
//                 return nums[i];
//             }
//             if (nums[i] != nums[i + 1]) {
//                 return nums[i];
//             }
//         }
//         return -1;

        // 2. the same as 1), just looping differently
//         if (nums.length == 1) {
//             return nums[0];
//         }
//         Arrays.sort(nums);
//         for (int i = 0, j = nums.length - 1; i < j; i += 2, j -= 2) {
//             if (nums[i] != nums[i + 1]) {
//                 return nums[i];
//             }
//             if (nums[j] != nums[j - 1]) {
//                 return nums[j];
//             }
//         }

//         return -1;

        // 4. first we add all numbers, then we sum them, the difference is the unique number (kind of over complicated)
//         Set<Integer> set = new HashSet<>();
//         for (int i = 0; i < nums.length; i++) {
//             set.add(nums[i]);
//         }

//         int s1 = 0;
//         Iterator<Integer> iterator = set.iterator();
//         while(iterator.hasNext()) {
//             s1 += iterator.next();
//         }

//         int s2 = 0;
//         for(int n : nums) {
//             s2 += n;
//         }

//         return 2 * s1 - s2;

        // 5. the same as 4, just fewer steps
//         Set<Integer> set = new HashSet<>();
//         int s2 = 0;
//         for (int i = 0; i < nums.length; i++) {
//             int num = nums[i];
//             set.add(num);
//             s2 += num;
//         }

//         int s1 = 0;
//         Iterator<Integer> iterator = set.iterator();
//         while(iterator.hasNext()) {
//             s1 += iterator.next();
//         }

//         return 2 * s1 - s2;
    }
}
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