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题目：Given a binary tree, determine if it is a valid binary search tree (BST).Assume a BST is defined as follows:The left subtree of a node contains on... 阅读全文
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题目：Given two numbers represented as strings, return multiplication of the numbers as a string.Note: The numbers can be arbitrarily large and are non-... 阅读全文
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题目：Follow up for "Remove Duplicates":What if duplicates are allowed at most twice?For example,Given sorted array A = [1,1,1,2,2,3],Your function shou... 阅读全文
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题目：Given an unsorted integer array, find the first missing positive integer.For example,Given [1,2,0] return 3,and [3,4,-1,1] return 2.Your algorithm... 阅读全文
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题目：The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed f... 阅读全文
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题目：Follow up for problem "Populating Next Right Pointers in Each Node".What if the given tree could be any binary tree? Would your previous solution ... 阅读全文
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题目：Given two binary strings, return their sum (also a binary string).For example,a = "11"b = "1"Return "100".题解：二进制加法都是从最低位（从右加到左）。所以对两个字符串要从最后一位开始加，... 阅读全文
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题目：The gray code is a binary numeral system where two successive values differ in only one bit.Given a non-negative integer n representing the total ... 阅读全文
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题目：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 ... 阅读全文
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题目：Determine whether an integer is a palindrome. Do this without extra space.click to show spoilers.Some hints:Could negative integers be palindromes... 阅读全文
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题目：Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique lo... 阅读全文