Description You are climbing a stair case. It takes n steps to reach to the top. Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top? Example Given an example n=3 , 1+1+1=2+1=1+2=3 return 3 又是一道动态规划类的题目,马上做…

You are climbing a stair case. It takes n steps to reach to the top. Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top? Note: Given n will be a positive integer. 题目分析:每次只能走1或2步,问n步的话有多少中走法???? 可以用动态规划和递归解…

这道题参考了这个网址: http://blog.csdn.net/u012490475/article/details/48845683 /* 首先考虑边界情况,当有1层时,有一种方法. 然后再看2层时,有1+1.和2+0,两种方法. 再看3层时,首先有两种选择:走一步或者走两步. 如果走两步,那后面还剩一步可走: 如果走一步,后面还剩两步可走,后面的方法即可等同于上面的2层情况. 即可归纳出用C(i) = j; 表示n层时有j种可能. C(1) = 1; C(2) = 2; C(3) = C(…

用栈模拟汉诺塔问题 描述 在经典的汉诺塔问题中,有 3 个塔和 N 个可用来堆砌成塔的不同大小的盘子.要求盘子必须按照从小到大的顺序从上往下堆 (如:任意一个盘子,其必须堆在比它大的盘子上面).同时,你必须满足以下限制条件: 每次只能移动一个盘子. 每个盘子从堆的顶部被移动后,只能置放于下一个堆中. 每个盘子只能放在比它大的盘子上面. 请写一段程序,实现将第一个堆的盘子移动到最后一个堆中. 样例 输入 3 输出 towers[0]: [] towers[1]: [] towers[2]: [2,…

斐波那契数列 描述 查找斐波纳契数列中第 N 个数. 所谓的斐波纳契数列是指: 前2个数是 0 和 1 . 第 i 个数是第 i-1 个数和第i-2 个数的和. 斐波纳契数列的前10个数字是: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34 ... 样例 给定 1,返回 0 给定 2,返回 1 给定 10,返回 34 思路 斐波那契数列算是一个很常见的典型递归问题了,直接就可以很轻松地根据定义写出代码: public int fibonacci(int n) { // write…

字符串置换 描述: 给定两个字符串,请设计一个方法来判定其中一个字符串是否为另一个字符串的置换. 置换的意思是,通过改变顺序可以使得两个字符串相等. 样例: "abc" 为 "cba" 的置换. "aabc" 不是 "abcc" 的置换. 思路: 误区是列出给定字符串的全部置换字符串然后再一一比对,这样肯定会超时. 根据题目给出的置换的概念和样例可以知道几个条件: 两个字符串长度相同. 每个字符串中的各个字符数量相同. 因此,…

Description Given a linked list, swap every two adjacent nodes and return its head. Example Given 1->2->3->4, you should return the list as 2->1->4->3. Challenge Your algorithm should use only constant space. You may not modify the value…

Description Cosine similarity is a measure of similarity between two vectors of an inner product space that measures the cosine of the angle between them. The cosine of 0° is 1, and it is less than 1 for any other angle. See wiki: Cosine Similarity H…

Description Given a boolean 2D matrix, 0 is represented as the sea, 1 is represented as the island. If two 1 is adjacent, we consider them in the same island. We only consider up/down/left/right adjacent. Find the number of islands. Example Given gra…

Description Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. Example The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]"…

Description Given a string s consists of upper/lower-case alphabets and empty space characters ' ', return the length of last word in the string. If the last word does not exist, return 0. A word is defined as a character sequence consists of non-spa…

Description The count-and-say sequence is the sequence of integers beginning as follows: 1, 11, 21, 1211, 111221, ... 1 is read off as "one 1" or 11. 11 is read off as "two 1s" or 21. 21 is read off as "one 2, then one 1" or …

Description Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases. Have you consider that the string might be empty? This is a good question to ask during an interview. For the purpose of this pr…

Description Reverse digits of an integer. Returns 0 when the reversed integer overflows (signed 32-bit integer). Example Given x = 123, return 321 Given x = -123, return -321 解题:注意别溢出即可,所以结果用long型变量保存,最后返回的时候强转成int型.代码如下: public class Solution { /**…

Description Given two binary strings, return their sum (also a binary string). Example a = 11 b = 1 Return 100 解题:二进制相加.我的思路是,先转成StringBuilder对象(reverse方法比较好用),因为相加是从最后开始,所以先用reverse方法倒转过来.和上一题类似,用carry变量表示进位,0为不进位,1为需要进一位.最后的结果再倒过来.具体细节标注在代码中,代码如下:…

Description Given a non-negative number represented as an array of digits, plus one to the number. The digits are stored such that the most significant digit is at the head of the list. Example Given [1,2,3] which represents 123, return [1,2,4]. Give…

Description Determine whether a Sudoku is valid. The Sudoku board could be partially filled, where empty cells are filled with the character .. A valid Sudoku board (partially filled) is not necessarily solvable. Only the filled cells need to be vali…

Description Given a binary tree, find all paths that sum of the nodes in the path equals to a given number target. A valid path is from root node to any of the leaf nodes. Example Given a binary tree, and target = 5: 1 / \ 2 4 / \ 2 3 return [ [1, 2,…

Description For the given binary tree, return a deep copy of it. Example Given a binary tree: 1 / \ 2 3 / \ 4 5 return the new binary tree with same structure and same value: 1 / \ 2 3 / \ 4 5 解题:链表复制.递归解法比较简单,代码如下: /** * Definition of TreeNode: * pu…

Description Partition an integers array into odd number first and even number second. Example Given [1, 2, 3, 4], return [1, 3, 2, 4] Challenge Do it in-place. 解题:将一个数组中的奇数和偶数分开,原地分开,不申请额外的空间.思路是,记录好最后一个奇数的后一个位置,也可以理解为偶数中的第一个·.遍历数组,如果当前数组中的元素是偶数,那么什么…

Description Implement an algorithm to delete a node in the middle of a singly linked list, given only access to that node. Example Linked list is 1->2->3->4, and given node 3, delete the node in place 1->2->4 解题:删除指定的结点.由于java没有delete 或者 fr…

Description Count how many 1 in binary representation of a 32-bit integer. Example Given 32, return 1 Given 5, return 2 Given 1023, return 9 Challenge If the integer is n bits with m 1 bits. Can you do it in O(m) time? 解题:很简单,但是要考虑范围的问题.代码如下: public…

Description There is a building of n floors. If an egg drops from the k th floor or above, it will break. If it's dropped from any floor below, it will not break. You're given two eggs, Find k while minimize the number of drops for the worst case. Re…

Description You have two very large binary trees: T1, with millions of nodes, and T2, with hundreds of nodes. Create an algorithm to decide if T2 is a subtree of T1. A tree T2 is a subtree of T1 if there exists a node n in T1 such that the subtree of…

Description In the classic problem of Towers of Hanoi, you have 3 towers and N disks of different sizes which can slide onto any tower. The puzzle starts with disks sorted in ascending order of size from top to bottom (i.e., each disk sits on top of…

Description Implement a method to perform basic string compression using the counts of repeated characters. For example, the string aabcccccaaa would become a2b1c5a3. If the "compressed" string would not become smaller than the original string,…

Climbing Stairs You are climbing a stair case. It takes n steps to reach to the top. Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top? 题目意思: 上楼梯.假设要n步到达楼梯的顶部.每一次你只能上一个或两级台阶,问要到达顶部一共有多少种方法? 解题思路: 真是太巧了!!我…

problem 746. Min Cost Climbing Stairs 题意: solution1:动态规划: 定义一个一维的dp数组,其中dp[i]表示爬到第i层的最小cost,然后来想dp[i]如何推导.思考一下如何才能到第i层呢?是不是只有两种可能性,一个是从第i-2层上直接跳上来,一个是从第i-1层上跳上来.不会再有别的方法,所以dp[i]只和前两层有关系,所以可以写做如下: dp[i] = min(dp[i- 2] + cost[i - 2], dp[i - 1] + cost[i…

Description Given a collection of intervals, merge all overlapping intervals. Example Given intervals => merged intervals: [ [ (1, 3), (1, 6), (2, 6), => (8, 10), (8, 10), (15, 18) (15, 18) ] ] Challenge O(n log n) time and O(1) extra space. 题意:给定一个…

170. Two Sum III - Data structure design[easy] Design and implement a TwoSum class. It should support the following operations:add and find. add - Add the number to an internal data structure.find - Find if there exists any pair of numbers which sum…