21. Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

19. Remove Nth Node From End of List

Given a linked list, remove the n-th node from the end of list and return its head.

17. Letter Combinations of a Phone Number

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.

15. 3Sum

Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

13. Roman to Integer

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

定义

贪心算法（又称贪婪算法）是指，在对问题求解时，总是做出在当前看来是最好的选择。也就是说，不从整体最优上加以考虑，他所做出的是在某种意义上的局部最优解。

贪心算法不是对所有问题都能得到整体最优解，关键是贪心策略的选择，选择的贪心策略必须具备无后效性，即某个状态以前的过程不会影响以后的状态，只与当前状态有关。

11. Container With Most Water

Given n non-negative integers a1, a2, …, an , where each represents a point at coordinate (i, ai). n vertical lines are drawn such that the two endpoints of line i is at (i, ai) and (i, 0). Find two lines, which together with x-axis forms a container, such that the container contains the most water.

定义

动态规划（dynamic programing）和分治法类似，都是通过组合子问题的解来求解原问题的解。（在经典排序算法中的二路归并排序和快速排序都用到了分而治之的思想-分治法）。

分治法是将原问题划分为没有交集，相互独立的子问题，并分别求解后再进行合并，求出原问题的解。

动态规划应用于子问题重叠的情况，即不同的子问题具有公共的子子问题。分治法会做许多不必要的工作，它会反复地求解那些公共子问题。动态规划算法对每个子问题只求解一次，将其解保存在一个表格中，从而无需每次求解一个子子问题时都需要重新计算。

9. Palindrome Number

Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward.

7. Reverse Integer

Given a 32-bit signed integer, reverse digits of an integer.