## leetcode Shuffle an Array

### leetcode Shuffle an Array

Shuffle a set of numbers without duplicates.

Example:

## leetcode Ransom Note

### leetcode Ransom Note

Given  an  arbitrary  ransom  note  string  and  another  string  containing  letters from  all  the  magazines,  write  a  function  that  will  return  true  if  the  ransom   note  can  be  constructed  from  the  magazines ;  otherwise,  it  will  return  false.

Each  letter  in  the  magazine  string  can  only  be  used  once  in  your  ransom  note.

Note:
You may assume that both strings contain only lowercase letters.

## leetcode Linked List Random Node

### leetcode Linked List Random Node

Given a singly linked list, return a random node’s value from the linked list. Each node must have the same probability of being chosen.

What if the linked list is extremely large and its length is unknown to you? Could you solve this efficiently without using extra space?

Example:

## leetcode Insert Delete GetRandom O(1) – Duplicates allowed

### leetcode Insert Delete GetRandom O(1) – Duplicates allowed

Design a data structure that supports all following operations in average O(1) time.

Note: Duplicate elements are allowed.

1. insert(val): Inserts an item val to the collection.
2. remove(val): Removes an item val from the collection if present.
3. getRandom: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains.

Example:

## leetcode Insert Delete GetRandom O(1)

### leetcode Insert Delete GetRandom O(1)

Design a data structure that supports all following operations in O(1) time.

1. insert(val): Inserts an item val to the set if not already present.
2. remove(val): Removes an item val from the set if present.
3. getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.

Example:

## leetcode Kth Smallest Element in a Sorted Matrix

### leetcode Kth Smallest Element in a Sorted Matrix

Given a n x n matrix where each of the rows and columns are sorted in ascending order, find the kth smallest element in the matrix.

Note that it is the kth smallest element in the sorted order, not the kth distinct element.

Example:

Note:
You may assume k is always valid, 1 ≤ k ≤ n2.

## leetcode Contains Duplicate 整理

• 217 Contains Duplicate
• 219 Contains Duplicate II
• 220 Contains Duplicate III

## leetcode Combination Sum IV

### leetcode Combination Sum IV

Given an integer array with all positive numbers and no duplicates, find the number of possible combinations that add up to a positive integer target.

Example:

What if negative numbers are allowed in the given array?
How does it change the problem?
What limitation we need to add to the question to allow negative numbers?

## leetcode K sum 整理

• 1. Two Sum
• 15. 3Sum
• 16. 3Sum Closest
• 18. 4Sum

## leetcode Wiggle Subsequence

### leetcode Wiggle Subsequence

A sequence of numbers is called a wiggle sequence if the differences between successive numbers strictly alternate between positive and negative. The first difference (if one exists) may be either positive or negative. A sequence with fewer than two elements is trivially a wiggle sequence.

For example, [1,7,4,9,2,5] is a wiggle sequence because the differences (6,-3,5,-7,3) are alternately positive and negative. In contrast,[1,4,7,2,5] and [1,7,4,5,5] are not wiggle sequences, the first because its first two differences are positive and the second because its last difference is zero.

Given a sequence of integers, return the length of the longest subsequence that is a wiggle sequence. A subsequence is obtained by deleting some number of elements (eventually, also zero) from the original sequence, leaving the remaining elements in their original order.

Examples: