## Leetcode - Pairs of Songs With Total Durations Divisible by 60

https://leetcode.com/problems/pairs-of-songs-with-total-durations-divisible-by-60/

In a list of songs, the `i`-th song has a duration of `time[i]` seconds.

Return the number of pairs of songs for which their total duration in seconds is divisible by `60`.  Formally, we want the number of indices `i < j` with `(time[i] + time[j]) % 60 == 0`.

Example 1:

```Input: [30,20,150,100,40]
Output: 3
Explanation: Three pairs have a total duration divisible by 60:
(time[0] = 30, time[2] = 150): total duration 180
(time[1] = 20, time[3] = 100): total duration 120
(time[1] = 20, time[4] = 40): total duration 60
```

## Leetcode - 4Sum

https://leetcode.com/problems/4sum/

Given an array `nums` of n integers and an integer `target`, are there elements a, b, c, and d in `nums` such that a + b + c + d = `target`? Find all unique quadruplets in the array which gives the sum of `target`.

Note:

The solution set must not contain duplicate quadruplets.

Example:

```Given array nums = [1, 0, -1, 0, -2, 2], and target = 0.

A solution set is:
[
[-1,  0, 0, 1],
[-2, -1, 1, 2],
[-2,  0, 0, 2]
]
```

## Leetcode - Two Sum III - Data structure design

https://leetcode.com/problems/two-sum-iii-data-structure-design/

Design and implement a TwoSum class. It should support the following operations: `add` and `find`.

`add` - Add the number to an internal data structure.
`find` - Find if there exists any pair of numbers which sum is equal to the value.

Example 1:

```add(1); add(3); add(5);
find(4) -> true
find(7) -> false
```

## Leetcode - Two Sum II - Input array is sorted

Given an array of integers that is already sorted in ascending order, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2.

Note:

• You may assume that each input would have exactly one solution and you may not use the same element twice.

Example:

```Input: numbers = [2,7,11,15], target = 9
Output: [1,2]
Explanation: The sum of 2 and 7 is 9. Therefore index1 = 1, index2 = 2.```

## Leetcode - subarray-sum-equals-k

https://leetcode.com/problems/subarray-sum-equals-k/

Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k.

Example 1:

```Input:nums = [1,1,1], k = 2
Output: 2
```

Note:

1. The length of the array is in range [1, 20,000].
2. The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].