类别归档：LeetCode

题目描述：

Given a linked list, determine if it has a cycle in it.

Follow up:

Can you solve it without using extra space?

题目大意：

给定一个链表，判断其中是否有环。

进一步思考：

你可以在不使用额外空间的条件下完成本题吗？

解题思路：

使用“快慢指针”法即可

fast指针每次向前运动两个节点，slow指针每次向前运动一个节点

如果fast和slow在链表的某处相遇，则说明链表中有环

Python代码（快慢指针）：

# Definition for singly-linked list ...

题目描述：

Given a sorted integer array without duplicates, return the summary of its ranges.

For example, given [0,1,2,4,5,7], return ["0->2","4->5","7"].

题目大意：

给定一组排好序且无重复的整数，返回整数范围的汇总。

例如给定数组 [0,1,2,4,5,7]， 返回 ["0->2","4->5" ...

题目描述：

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some ...

题目描述：

Invert a binary tree.

     4
   /   \
  2     7
 / \    / \
1   3   6  9

to

     4
   /   \
  7     2
 / \   / \
9   6  3  1

Trivia:

This problem was inspired by this original tweet by Max Howell:

Google: 90% of our engineers use the software ...

题目描述：

Implement the following operations of a stack using queues.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
empty() -- Return whether the stack is empty.

Notes:
You ...