105 lines
2.3 KiB
Markdown
105 lines
2.3 KiB
Markdown
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# Leetcode Two Sum
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#### 2022-06-10
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---
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##### Data structures:
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#DS #array #map #unordered_map
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##### Algorithms:
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#algorithm
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##### Difficulty:
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#coding_problem #leetcode #difficulty-easy
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##### Related topics:
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##### Links:
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- [Link to problem](https://leetcode.com/problems/two-sum)
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- [O(n) Solution](https://leetcode.com/problems/two-sum/discuss/13/Accepted-C++-O(n)-Solution/263)
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___
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### Problem
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Given an array of integers `nums` and an integer `target`, return _indices of the two numbers such that they add up to `target`_.
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You may assume that each input would have **_exactly_ one solution**, and you may not use the _same_ element twice.
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You can return the answer in any order.
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#### Examples
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Example 1:
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```
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Input: nums = [2,7,11,15], target = 9
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Output: [0,1]
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Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].
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```
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Example 2:
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```
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Input: nums = [3,2,4], target = 6
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Output: [1,2]
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```
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Example 3:
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```
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Input: nums = [3,3], target = 6
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Output: [0,1]
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```
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#### Constraints
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- `2 <= nums.length <= 104`
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- `-109 <= nums[i] <= 109`
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- `-109 <= target <= 109`
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- **Only one valid answer exists.**
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### Thoughts
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Firstly, I think up an easy O(n^2) solution, by using a nested loop,
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But the best solution utilizes an __[[cpp_std_unordered_map]]__.
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> [!tips]
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> Use **[[cpp_std_unordered_map | unordered map]]** to create a hash table, it has O(1) in search, delete and insert. #tip
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### Solution
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O(n^2) solution
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```cpp
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class Solution {
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public:
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vector<int> twoSum(vector<int>& nums, int target) {
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vector<int> pair;
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for (int i = 0; i < nums.size(); i++) {
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for (int j = i + 1; j < nums.size(); j++) {
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if (nums[i] + nums[j] == target) {
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pair.insert(pair.begin(), i);
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pair.insert(pair.begin(), j);
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return pair;
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}
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}
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}
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return pair;
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}
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};
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```
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O(n) solution
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```cpp
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class Solution {
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public:
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vector<int> twoSum(vector<int>& nums, int target) {
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// use unordered_map here.
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unordered_map<int, int> umap;
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for (int i = 0; i < nums.size(); i++) {
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auto search = umap.find(target - nums[i]);
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if ( search != umap.end()) {
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return vector<int> {i, search->second};
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}
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umap[nums[i]] = i;
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}
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return vector<int> {0, 0};
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}
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};
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```
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