110. Balanced Binary Tree

just do it

131. Palindrome Partitioning

brute force recursion

134. Gas Station

find the travel length l of starting point i, the travel length of starting point in [i .. i + j] are no bigger than l

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if the total gas is greater than costs, there must be a starting point. start i and start point from 0, each time curr reaches a negative value, set starting point to i + 1

139. Word Break

recursion+memorization or dp

146. LRU Cache

HashMap<Key, Node> & DoublyLinkedList<Value>

150. Evaluate Reverse Polish Notation

151. Reverse Words in a String

先反转整个字符串 in-place，再反转每个单词 in-place

162. Find Peak Element

binary search, whether m is at a descending slope or not

163. Missing Ranges

let lo = lower - 1 initially, then compare each value with lo then update lo, finally check lo with upper

166. Fraction to Recurring Decimal

using long division(the way we do division by hand) and a hash map to check if the current numerator has once appeared, if so, we found the repeating range. the first time the current numerator is smaller than the denomenater, place the comma

167. Two Sum II - Input array is sorted

two pointers

170. Two Sum III - Data structure design

it seems that there has to be one operation's time complexity is O(n) and the other is O(1).
then it's trivial

172. Factorial Trailing Zeroes

every zero comes from factors 5 * 2, there are plenty of 2s, so the key is to find how many 5-factors.

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community solution

190. Reverse Bits

get every pos using bit op then construct a new one

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every time you right shift the original number, left shift the resulting number then add 1(or 0)

199. Binary Tree Right Side View

find right most of each level

200. Number of Islands

dfd || bfs

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union find