Problem

Given an array of size n, find the majority element. The majority element is the element that appears more than ⌊ n/2 ⌋ times.

You may assume that the array is non-empty and the majority element always exist in the array.

找到数组中重复出现次数超过数组长度⌊ n/2 ⌋ 的元素

Problem

Given a positive integer, return its corresponding column title as appear in an Excel sheet.

For example:

    1 -> A
    2 -> B
    3 -> C
    ...
    26 -> Z
    27 -> AA
    28 -> AB

即生成Excel表格的表头

Problem

Given an array of integers that is already sorted in ascending order, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution and you may not use the same element twice.

Input: numbers={2, 7, 11, 15}, target=9 Output: index1=1, index2=2

即找到数组中两个元素的和等于给定数字的元素，数组已排序。

Problem

Write a program to find the node at which the intersection of two singly linked lists begins.

For example, the following two linked lists:

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

begin to intersect at node c1.

Notes:

If the two linked lists have no intersection at all, return null. The linked lists must retain their original structure after the function returns. You may assume there are no cycles anywhere in the entire linked structure. Your code should preferably run in O(n) time and use only O(1) memory.

即找到两个链表相交的第一个元素。要求不改变链表结构，O（n)时间， O(1)内存

Problem

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

push(x) – Push element x onto stack. pop() – Removes the element on top of the stack. top() – Get the top element. getMin() – Retrieve the minimum element in the stack. Example:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> Returns -3.
minStack.pop();
minStack.top();      --> Returns 0.
minStack.getMin();   --> Returns -2.

即自定义一个栈，除了支持栈的基本操作外，支持一个额外的getMin返回当前栈内元素最小值的操作。

Problem

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Note: Do not modify the linked list.

Follow up: Can you solve it without using extra space?

找到链表中环的入口节点，不开辟额外空间

Problem

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

Follow up: Can you solve it without using extra space?

判断一个链表中是否存在 环，不开辟额外的空间

Problem

Given an array of integers, every element appears three times except for one, which appears exactly once. Find that single one.

Note: Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

一个整型数组，除一个元素外，其余的元素都出现三次，要求不用额外内存，线性时间内找到这个元素

Problem

Given an array of integers, every element appears twice except for one. Find that single one.

Note: Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

一个整型数组，除一个元素外，其余的元素都出现两次，要求不用额外内存，线性时间内找到这个元素

Problem

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example, “A man, a plan, a canal: Panama” is a palindrome. “race a car” is not a palindrome.

Note: Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

即判断一个字符串的字符部分是不是回环字符串。即出去非字母，数字以外的字符，剩下的部分满足正反相同。