类别归档：LeetCode

题目描述：

Implement a trie with insert, search, and startsWith methods.

Note:
You may assume that all inputs are consist of lowercase letters a-z.

题目大意：

实现字典树，包含插入，查找和前缀查找方法。

注意：

你可以假设所有的输入只包含小写字母a-z

解题思路：

本题考查字典树数据结构的基础知识。

Trie使用孩子表示法存储，TrieNode为字典树的节点，包含属性childs和isWord。

其中childs为dict，存储当前节点的后代节点；isWord为布尔值，表示当前节点是否存储了一个单词。

Python代码：

class TrieNode:
    # Initialize your data structure ...

题目描述：

There are a total of n courses you have to take, labeled from 0 to n - 1.

Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed ...

题目描述：

Reverse a singly linked list.

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

题目大意：

单链表逆置

提示：
用递归和迭代方式分别实现。

解题思路：

链表基本操作，递归方式参考：https://leetcode.com/discuss/34474/in-place-iterative-and-recursive-java-solution

Python代码：

迭代版本：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, ...

题目描述：

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 ...

题目描述：

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

Hint: The number n could be in the order of 100,000 to 5,000,000.

References:

How Many Primes Are There? (https://primes.utm.edu/howmany.html)

Sieve of Eratosthenes (http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes)

题目大意：

统计小于非负整数n的素数的个数

提示：n的范围是100,000到5,000,000 ...