136. Single Number

https://leetcode.com/problems/single-number

Problem

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

Solution

1. $O(N)$ Space using Hash Map

class Solution {
public:
    int singleNumber(vector<int>& nums) {
        unordered_map<int, int> count_of;
        for (const auto &n : nums) ++count_of[n];
        for (const auto &pair : count_of) {
            if (pair.second == 1) return pair.first;
        }
        return -1;
    }
};

2. $O(1)$ Space using XOR bitwise

class Solution {
public:
    int singleNumber(vector<int>& nums) {
        int ans = 0;
        for (const auto& n: nums) ans ^= n;
        return ans;
    }
};

• #hash

• #bitwise