# Algorithm problem list
## level 1 easy
- https://leetcode.com/problems/check-if-n-and-its-double-exist/
- hash table =>T: O(N), S: O(N)
- binary search => T: O(NlogN), S: O(1)
- corner case: value 0
- non-unique value
:::info
- Test case

Given an integer array A, write a function to check if there exists different index i and j such that A[i] = 2*A[j]
if exist such pair, return true
otherwise, return false

[10,2,5,3]
[10,2,0,0,5,3]
[-2,-1,0,1,3]

:::
- 單敗淘汰賽, 比出冠軍最少需要多少場, 可不fair
- O(logN) => divide+remain 計算
- O(1) => N-1

- https://leetcode.com/problems/redistribute-characters-to-make-all-strings-equal/
- hash table
- https://leetcode.com/problems/minimum-operations-to-make-the-array-increasing/
- sum of difference of two adjacent values

- https://leetcode.com/problems/best-meeting-point/
- 1維版本
- naive solution O(N^2)
- opt solution O(N)
- follow up, 2維版本 (level 3-4)
- [1,0,1,0,0,0,1,0,1] => answer = 12
- follow up, 可容納多個人情況
- [3,0,2,1,3,0,2,0,3]
## level 2
- https://leetcode.com/problems/satisfiability-of-equality-equations/
- union find

  Input: ["a==b","b!=a"]
  Input: ["b==a","a==b"]
  Input: ["a==b","c!=a", "a==d", "c!=d"]
  Input: ["a==b","b==c", "c!=a"]
label = 1000
equation = 1e5

- https://leetcode.com/problems/tuple-with-same-product/
- hast table + math

- https://leetcode.com/problems/max-consecutive-ones-iii
- two pointer
- O(N)
- binary search
- O(NlogN)

- https://codeforces.com/contest/1625/problem/A
- 思考題
- bitwise operation

Given an integer array A, write a function to find a interger X (may not in A)
1. X has minimum sum of Hamming distance to all other elements in array
2. X is as small as possible

Hamming distance is the number of positions at which the corresponding bits are different.

18 9 21
ans = 17

10010
01001
10101

answer
10001

## level 3
- https://leetcode.com/problems/maximum-subarray-min-product/
- prev smaller + next smaller + range sum
- the same as max [histogram](https://leetcode.com/problems/largest-rectangle-in-histogram/)
- https://leetcode.com/problems/largest-submatrix-with-rearrangements/
- rolling rectangle area
- O(M\*N)

- https://leetcode.com/problems/longest-string-chain/
- dfs + dp

["a","b","ba","bca","bda","bdca"]
["a","ba","bda","bdca"]
1 <= words.length <= 1000
1 <= words[i].length <= 16
words[i] only consists of lowercase English letters.

## level 4