Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Example 1:
Input: k = 3, n = 7
Output:
[[1,2,4]]

Example 2:
Input: k = 3, n = 9
Output:
[[1,2,6], [1,3,5], [2,3,4]]

Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

For example,
Given [3,2,1,5,6,4] and k = 2, return 5.

Note:
You may assume k is always valid, 1 ≤ k ≤ array’s length.

Given a string S, you are allowed to convert it to a palindrome by adding characters in front of it. Find and return the shortest palindrome you can find by performing this transformation.

For example:
Given “aacecaaa”, return “aaacecaaa”.
Given “abcd”, return “dcbabcd”.

Given an array of n positive integers and a positive integer s, find the minimal length of a contiguous subarray of which the sum ≥ s. If there isn’t one, return 0 instead.

For example, given the array [2,3,1,2,4,3] and s = 7,
the subarray [4,3] has the minimal length under the problem constraint.

Reverse a singly linked list.

Hint:
A linked list can be reversed either iteratively(迭代) or recursively(递归).Could you implement both ?

Given two strings s and t, determine if they are isomorphic.
Two strings are isomorphic if the characters in s can be replaced to get t.

All occurrences of a character must be replaced with another character while preserving the order of characters.No two characters may map to the same character but a character may map to itself.

For example,
Given “egg”, “add”, return true.
Given “foo”, “bar”, return false.
Given “paper”, “title”, return true.
Note:
You may assume both s and t have the same length.

Count the number of prime numbers less than a non - negative number, n.

题意：

　　计算小于n的非负数的质数的个数。

Remove all elements from a linked list of integers that have value val.

Example
Given : 1 – > 2 – > 6 – > 3 – > 4 – > 5 – > 6, val = 6
Return: 1 – > 2 – > 3 – > 4 – > 5

Write an algorithm to determine if a number is “happy”.

A happy number is a number defined by the following process : Starting with any positive integer, replace the number by the sum of the squares of its digits, and repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1. Those numbers for which this process ends in 1 are happy numbers.

Example : 19 is a happy number
​ 1^2 + 9^2 = 82
​ 8^2 + 2^2 = 68
​ 6^2 + 8^2 = 100
​ 1^2 + 0^2 + 0^2 = 1

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

1
2
3
4
5
6
7
8
9

For example:
Given the following binary tree,

   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---
You should return [1, 3, 4].